KYIV, Ukraine -- Ukrainian forces on Tuesday battled continuing Russian efforts to occupy Mariupol and claimed to have retaken a strategic suburb of Kyiv, mounting a defense so dogged that it is stoking fears that Russian President Vladimir Putin will escalate the war to new heights.
"Putin's back is against the wall," said U.S. President Joe Biden, who is heading to Europe this week to meet with allies. "And the more his back is against the wall, the greater the severity of the tactics he may employ."
Biden reiterated accusations that Putin is thinking about resorting to the use of chemical or biological weapons, though Pentagon spokesman John Kirby said the U.S. has seen no evidence to suggest that such an escalation is imminent.
Biden's warning came as attacks continued in and around Kyiv and Mariupol.
Ukrainian President Volodymyr Zelenskyy accused Russian forces of not only blocking a humanitarian convoy trying to take desperately needed aid to Mariupol but also of seizing what another Ukrainian official said were 15 of the bus drivers and rescue workers on the aid mission, along with their vehicles.
Zelenskyy said the Russians had agreed to the route ahead of time.
"We are trying to organize stable humanitarian corridors for Mariupol residents, but almost all of our attempts, unfortunately, are foiled by the Russian occupiers, by shelling or deliberate terror," he said in his nightly video address to the nation.
Russian forces have started shelling Mariupol from the sea, a senior Pentagon official said, an escalation of Russia's assault on the southern port city.
There were up to seven Russian warships in the Sea of Azov, according to a count from the official, who added that the city is an "anchor" for the Russian effort to overtake the region.
Elsewhere, at least seven forest fires have broken out around the Chernobyl nuclear power plant, Ukraine's parliament said Monday, raising fears that they could spread radiation. Ukrainian officials and firefighters could not carry out their usual functions in the area to extinguish the fires because of Russian control of the plant.
Explosions and bursts of gunfire shook Kyiv, and heavy artillery fire could be heard from the northwest, where Russia has sought to encircle and capture several of the capital's suburban areas.
Early Tuesday, Ukrainian troops drove Russian forces from the Kyiv suburb of Makariv after a fierce battle, Ukraine's Defense Ministry said. The regained territory allowed Ukrainian forces to take control of a key highway and block Russian troops from surrounding Kyiv from the northwest.
A video posted by Ukrainian police showed them surveying damage in Makariv, including to the town's police station, which an officer said took a direct hit to its roof. The police drove past destroyed residential buildings and along a road pocked by shelling. The town appeared all but deserted.
Officials warned, though, that the outlook in Ukraine is far from rosy.
The Defense Ministry said Russian forces partially took the northwest suburbs of Bucha, Hostomel and Irpin, some of which have been under attack almost since Russia invaded nearly a month ago.
A Western official, speaking on condition of anonymity to discuss military assessments, said Ukrainian resistance has brought much of Russia's advance to a halt but has not sent Moscow's forces into retreat.
"This war will not end easily or rapidly," said U.S. national security adviser Jake Sullivan. Still, he added, "Whether Russia takes a city or takes a town or takes more territory, they are never going to be able to achieve the purpose that they set out, which was to subjugate this country, to bring this country to heel, because the Ukrainian people have made very clear that they will not be subjugated no matter what it takes."
"We have seen indications that the Ukrainians are going a bit more on the offensive now," Kirby told reporters separately in Washington. He said that was particularly true in southern Ukraine, including near Kherson, where "they have tried to regain territory."
Asked on CNN what Putin had achieved in Ukraine, Kremlin spokesman Dmitry Peskov said: "Well, first of all, not yet. He hasn't achieved yet." But he insisted that the military operation was going "strictly in accordance with the plans and purposes that were established beforehand."
Putin's aims remain to "get rid of the military potential of Ukraine" and to "ensure that Ukraine changes from an anti-Russian center to a neutral country," Peskov said.
CONFLICT'S TOLL
Russia's far stronger, bigger military has many Western military experts warning against overconfidence in Ukraine's long-term odds. Russia's practice in past wars in Chechnya and Syria was to grind down resistance with strikes that flattened cities, killed countless civilians and sent millions of people fleeing.
But Russian forces appeared unprepared and have often performed badly against Ukrainian resistance. The U.S. estimates that Russia has lost a bit more than 10% of the overall combat capability it had at the start of the fight, including troops and tanks and other materiel.
Western officials say Russian forces are facing serious shortages of food, fuel and cold weather gear, leaving some soldiers suffering from frostbite.
The invasion has driven more than 10 million people from their homes, almost a quarter of Ukraine's population, according to the United Nations.
Thousands of civilians are believed to have died. Estimates of Russian military casualties vary widely, but even conservative figures by Western officials are in the low thousands.
On Monday, Russia's pro-Kremlin Komsomolskaya Pravda newspaper, citing the Defense Ministry, reported that almost 10,000 Russian soldiers had been killed. The report was quickly removed, and the newspaper blamed hackers. The Kremlin refused to comment.
The Western official said the figure is "a reasonable estimate."
Putin's troops are facing unexpectedly stiff resistance that has left the bulk of Moscow's ground forces miles from the center of Kyiv, and they are making slow progress on apparent efforts to cut off fighters in eastern Ukraine. The Russians are increasingly concentrating their air power and artillery on Ukraine's cities and civilians.
Talks to end the fighting have continued by video. Zelenskyy said negotiations with Russia are going "step by step, but they are going forward."
U.N. Secretary-General Antonio Guterres said he saw progress in the talks.
"From my outreach with various actors, elements of diplomatic progress are coming into view on several key issues," and the gains are enough to end hostilities, he said. He gave no details.
The Western official, though, said that there were no signs Moscow was ready to compromise.
In the most recent update from Mariupol officials, they said March 15 that at least 2,300 people had died in the siege. Accounts from the city suggest the true toll is much higher, with bodies lying uncollected. Airstrikes over the past week destroyed a theater and an art school where many civilians were taking shelter.
Zelenskyy, in his address, said more than 7,000 people were evacuated from Mariupol on Tuesday.
Before the war, 430,000 people lived in Mariupol. But about 100,000 remain in the city "in inhuman conditions, under a full blockade, without food, without water, without medicine and under constant shelling, under constant bombardment," he said.
The Red Cross confirmed that a humanitarian aid convoy trying to reach the city with desperately needed supplies had not been able to enter.
Perched on the Sea of Azov, Mariupol is a crucial port for Ukraine and lies along a stretch of territory between Russia and Crimea.
The siege has cut the city off from the sea and allowed Russia to establish a land corridor to Crimea. But it's not clear how much of the city Russia holds, with fleeing residents saying that fighting continues street by street.
Ukraine's Defense Ministry said that troops defending the city had destroyed a Russian patrol boat and electronic warfare complex.
Beyond the human toll, the war has shaken the post-Cold War global security consensus, imperiled the world supply of key crops and raised worries it could set off a nuclear accident.
As part of a series of addresses to foreign legislatures, Zelenskyy urged Italian lawmakers to strengthen sanctions against Moscow, noting that many wealthy Russians have homes in the country.
"Don't be a resort for murderers," he said from Kyiv.
MORE SANCTIONS
Biden plans to announce new sanctions against Russia on Thursday while in Brussels for meetings with NATO and European allies, according to a top national security aide.
The U.S. president, who will take part in a special meeting of NATO and address the European Council summit, is also expected to underscore efforts to enforce the avalanche of existing sanctions already announced by the U.S. and its allies.
"He will join our partners in imposing further sanctions on Russia and tightening the existing sanctions to crack down on evasion and to ensure robust enforcement," said Sullivan, who declined to further preview the new sanctions the president will announce.
Sullivan added that Biden will also "announce joint action on enhancing European energy security and reducing Europe's dependence on Russian gas."
Biden is traveling to Brussels and Poland to push for continued unity among Western allies as Russia presses on with its invasion of Ukraine. Biden will land in Brussels tonight.
In Poland, Biden will meet with Polish President Andrzej Duda, who has requested further U.S. aid and a stepped-up military presence on NATO's eastern flank as the war grinds on. The U.S. has already more than doubled its regular presence of more than 4,000 U.S. troops. Currently, there are about 10,000 U.S. troops in Poland.
Estonia, Latvia, Lithuania and Romania have also called for a greater NATO or U.S. military presence in recent weeks.
Sullivan suggested that could be coming soon as Biden plans to have talks "on longer-term adjustments to NATO force posture on the eastern flank."
"We feel that it is the right place for him to go to be able to see troops, to be able to see humanitarian experts and to be able to meet with a frontline and very vulnerable ally," Sullivan said of Biden's visit to Poland.
Talks on troop adjustments are already underway.
Biden and NATO have said repeatedly that while the U.S. and the alliance will provide weapons and other defensive support to non-NATO member Ukraine, they are determined to avoid any escalation with Russia.
Polish leaders have called for a Western peacekeeping mission to intervene in Ukraine, a step that the U.S. and other allies worry could lead to a broadening of the war.
APPEAL TO ITALY
In a speech by video link that was greeted by two standing ovations, Zelenskyy warned Italy's Parliament on Tuesday that the invasion of his country could become a stepping stone for Russian forces seeking to enter Europe, adding that famine would strike in parts of the world if Ukrainian farmers were unable to continue their work.
"Ukraine is the gate for the Russian army -- they want to enter Europe," he said. "But barbarity should not enter."
Zelenskyy warned that "famine was approaching for several countries" that depended on Ukrainian corn, oil and wheat, including Italy's "neighbors across the sea," referring to some North African nations.
"How can we sow under the strikes of Russian artillery? How can we cultivate when our enemy destroys our fields and our fuel?" he said.
In response, Prime Minister Mario Draghi of Italy said the country wanted Ukraine to join the European Union and praised the "heroic" resistance of the Ukrainians against the "ferociousness" of Putin. He also said Italy would respond with military aid.
"We are ready to do more," Draghi said. "In front of inhumanity, Italy has no intention of looking away."
Zelenskyy thanked Italy for taking in children and women fleeing the war, in private homes and hospitals. He said 117 children have been killed in the war so far.
"And this is not the final number," he said, describing the devastation caused by the weekslong conflict.
He urged Italy to freeze Russian assets, mentioning the Scheherazade -- a luxury yacht located in the Tuscan coastal town of Marina di Carrara that The New York Times reported to be potentially linked to Putin.
Zelenskyy also told Italian lawmakers that he had a morning conversation with Pope Francis and that the pontiff understood the Ukrainians' desire for peace and need to defend themselves.
Information for this article was contributed by Nebi Qena, Cara Anna, Yuras Karmanau, Aamer Madhani, Robert Burns, Colleen Long, Lorne Cook and staff members of The Associated Press; by Gaia Pianigiani of The New York Times; and by David L. Stern, Hannah Allam, Adela Suliman, Ellen Francis, Karoun Demirjian, Annabelle Chapman, Annabelle Timsit, Amy Cheng, Stefano Pitrelli, Miriam Berger, Rachel Pannett, Paulina Firozi, Dan Lamothe and Matt Viser of The Washington Post.
A woman waits for a transport after fleeing the war from neighbouring Ukraine at a railway station in Przemysl, Poland, on Tuesday, March 22, 2022. (AP Photo/Sergei Grits)
People roam around a used-books street market in downtown Lviv, western Ukraine, Tuesday, March 22, 2022. The U.N. refugee agency says more than 3.5 million people have fled Ukraine since Russia's invasion, passing another milestone in an exodus that has led to Europe's worst refugee crisis since World War II. (AP Photo/Bernat Armangue)
A couple kisses in downtown Lviv, western Ukraine, Tuesday, March 22, 2022. The U.N. refugee agency says more than 3.5 million people have fled Ukraine since Russia's invasion, passing another milestone in an exodus that has led to Europe's worst refugee crisis since World War II. (AP Photo/Bernat Armangue)
Ukrainian President Volodymyr Zelenskyy appears on the television inside a bar in downtown Lviv, western Ukraine, Tuesday, March 22, 2022. Ukrainian forces fought off continuing Russian efforts to occupy Mariupol and claimed to have retaken a strategic suburb of Kyiv on Tuesday, mounting a defense so dogged that it is stoking fears Russia's Vladimir Putin will escalate the war to new heights. (AP Photo/Bernat Armangue)
Ukrainian serviceman checks documents of a driver in Kharkiv, Ukraine, Tuesday, March 22, 2022. (AP Photo/Andrew Marienko)
Refugees from Lviv, Ukraine, sit in a Red Cross headquarters in Rome, Tuesday, March 22, 2022. A convoy of Red Cross vehicles carrying 80 people, mostly elderly and sick arrived Tuesday in Rome. (AP Photo/Alessandra Tarantino)
A family walks next to anti- tank barricades in Odesa, southern Ukraine, on Tuesday, March 22, 2022.(AP Photo/Petros Giannakouris)
Ukrainian serviceman has a rest on his position in Kharkiv, Ukraine, Tuesday, March 22, 2022. (AP Photo/Andrew Marienko)
A car destroyed by shelling is seen in a street in Kharkiv, Ukraine, Tuesday, March 22, 2022. (AP Photo/Andrew Marienko)