KIEV, Ukraine -- Ukraine's president said Sunday that intercepted radio and telephone conversations prove that Russia-backed separatists were responsible for firing the rockets that pounded the southeastern city of Mariupol and killed at least 30 people.
The attacks on Mariupol, a strategically situated port city that had been relatively quiet for months, alarmed the West.
Putting the blame on Moscow, President Barack Obama said the U.S. will work with its European partners to "ratchet up the pressure on Russia."
European Union foreign affairs chief Federica Mogherini later announced that EU foreign ministers will hold an "extraordinary" meeting in Brussels on Thursday to discuss Ukraine.
The U.S. and the EU warned that Russia may face repercussions after the Saturday attacks. The projectiles were launched from rebel-held territory, the U.S., NATO, and the Organization for Security and Cooperation in Europe said. The separatists blamed government forces.
Russian Foreign Minister Sergey Lavrov spoke separately with Mogherini and U.S. Secretary of State John Kerry, telling them the Ukrainian government bore responsibility for the latest military escalation, according to statements released by his ministry. Lavrov did not, however, directly address who had carried out the attack on Mariupol and said it should be investigated.
Separatist leader Alexander Zakharchenko initially announced that his forces had begun an offensive on the government-controlled city of Mariupol. But after the extent of civilian casualties became known, he backtracked and blamed Ukrainian forces for Saturday's carnage.
The rocket attack came a day after the rebels rejected a peace deal and announced they were going on a multipronged offensive against the Kiev government in a bid to seize more territory. The rebel stance has upended European attempts to mediate an end to the fighting in eastern Ukraine.
"The intercepted radio and telephone conversations, which were given to me by Ukraine's security services, irrefutably prove that the attack was conducted by the terrorists, who, unfortunately, are supported by Russia," President Petro Poroshenko said during an emergency meeting of his Security Council.
The situation along the front line is "stable" and Ukraine is reinforcing its positions, Defense Minister Stepan Poltorak said at the meeting.
Poroshenko urged a "reliable" cease-fire, the pullback of troops, the closing of the country's border with Russia and a return to the agreements signed earlier.
A peace deal signed in September in Minsk, Belarus, envisaged a cease-fire and a pullout of heavy weapons from a division line in eastern Ukraine, but both sides have repeatedly violated the pact.
A meeting with the government would be "inappropriate" in the current conditions, said Vladislav Deynego, a negotiator for the self-proclaimed Luhansk People's Republic, according to the Luhansk Information Center news website.
In Mariupol on Sunday, emergency workers disposed of rocket fragments left by the attack. Police said two unexploded rockets were found in a bank and an apartment building.
United Nations refugee agency workers handed out blankets to people left homeless or without heat because of the shelling, which hit schools, homes and shops.
"The city is in shock," Mariupol resident Yelena Khorshenko said. "The streets are empty, and people are boarding up their windows and preparing for the worst."
Mariupol lies between Russia and the Russian-annexed Crimean Peninsula. Heavy fighting in the region in the fall raised fears that the separatists would try to capture the city to forge a land link between the two.
In Kiev, hundreds gathered on the central square in memory of those who died in Mariupol. In addition to the 30 people killed, 95 people were wounded.
The attacks on Mariupol "appear to have been launched indiscriminately into civilian areas, which would constitute a violation of international humanitarian law," U.N. Secretary-General Ban Ki-moon's office said in a statement dated Saturday.
During a visit to New Delhi, Obama said the U.S. is "deeply concerned about the latest break in the cease-fire and the aggression that these separatists with Russian backing, Russian equipment, Russian financing, Russian training and Russian troops are conducting."
"And we will continue to take the approach that we've taken in the past, which is to ratchet up the pressure on Russia, and I will look at all additional options that are available to us short of military confrontation and try to address this issue," he said.
Obama said the U.S. will work "in close consultation with our international partners, and particularly European partners, to ensure that they stay in lock-step with us on this issue."
Ukraine, the U.S. and its allies accuse Russia of supporting the rebels with hardware, cash and thousands of troops, accusations the Kremlin has repeatedly denied. Russia says the government in Kiev is waging war against its own citizens and discriminating against Russian speakers, who make up the majority of the populations of Donetsk and Luhansk.
The U.S. and EU have imposed sanctions on Russian individuals, businesses and entire sectors of the economy over Russia's annexation of Crimea and role in fomenting the conflict in eastern Ukraine.
Lavrov, in the calls with Kerry and Mogherini, attributed the latest violence to stepped-up operations by the Ukrainian military. He urged the West to pressure Ukraine to engage in comprehensive talks for a political solution to the conflict, the ministry statements said.
"Lavrov stressed Russia's willingness to do anything it can to prompt parties to reach a peaceful resolution," the ministry said. "Kiev is avoiding" dialogue with the rebels "in every way, clearly having taken the course of suppression by force in Southeast Ukraine. Lavrov again urged the U.S. to use its influence on Ukrainian authorities to stop them betting on the military scenario."
Kerry told Lavrov that the U.S. was ready to participate in serious efforts to settle the conflict, but he made clear that "Russia will be judged by its actions and that the costs to Russia will only increase if attacks continue," the State Department said.
German Chancellor Angela Merkel called Poroshenko and Russian President Vladimir Putin on Sunday. Merkel expressed her condolences to Poroshenko for the latest civilian deaths, said her spokesman, Steffan Seabed.
"During her call with President Putin, the chancellor urged him to avoid a further escalation and to exert influence on the separatists in order to achieve the implementation of the Minsk agreement," the spokesman said.
Fighting also has intensified for control over Debaltseve, a government-held town and railway hub about 30 miles east of Donetsk, the main city under separatist control.
Ukraine military spokesman Andriy Lysenko said rebel shelling had killed an undetermined number of civilians and damaged 60 residential buildings. He said there was no electricity or heat in the city of 25,000 people.
The death toll in the conflict, which has brought tensions between Russia and its Cold War-era foes to their highest pitch since the fall of communism a quarter-century ago, rose to more than 5,000 since it began last April, according to the U.N. Jan. 13-Wednesday was "the most deadly period" in the conflict since the truce was signed, the U.N. said Friday.
Russia continues to send people and equipment across the border, Lysenko told reporters in Kiev on Sunday. Four government soldiers died and 17 were wounded in the previous 24 hours, he said. Government forces killed about 600 separatists and destroyed 14 tanks from Jan. 17 through Thursday, he said. The numbers were impossible to verify.
Information for this article was contributed by Yuras Karmanau, Evgeniy Maloletka, Julie Pace, Lynn Berry, Raf Casert and Frank Jordans of The Associated Press and by Daryna Krasnolutska, Aliaksandr Kudrytski, Sangwon Yoon, Kateryna Choursina, Volodymyr Verbyany, Anna Andrianova, Angela Greiling Keane, Alex Morales, Fergal O'Brien and Brian Parkin of Bloomberg News.
A Section on 01/26/2015