MOSCOW - Two suicide bombings in as many days have killed 31 people and raised concerns that Islamic militants have begun a terrorist campaign in Russia that could stretch into the Sochi Olympics in February. Russian and international Olympic officials insisted that the site of the games, protected by layers of security, is completely safe.

The attacks in Volgograd, about 400 miles from Sochi, reflected the Kremlin’s inability to uproot Islamist insurgents in the Caucasus who have vowed to derail the games, the pet project of President Vladimir Putin.

“I am concerned,” U.S. speed skater Jilleanne Rookard said. “I’m scared their security may be involved. I don’t know if I necessarily trust their security forces. But they don’t want a national embarrassment, either. I use that thought to relieve some of my worry. I’m sure they want to save their image and their pride.”

No one has claimed responsibility for Sunday’s blast at the Volgograd railway station or Monday’s bus explosion in the city, but they came only months after Chechen rebel leader Doku Umarov threatened new attacks on civilian targets in Russia, including the Olympics.

In addition to the dead, the bombings wounded 104 people, according to Russia’s Health Ministry. As of late Monday, 58 remained hospitalized, many in grave condition.

Suicide bombings have rocked Russia for years, but the insurgents seeking to create an Islamic state largely have confined their attacks to the North Caucasus region in recent years. The blasts in Volgograd signaled that militants want to show their reach outside their native region.

Matthew Clements, an analyst at Jane’s Information Group, said Caucasus militants could be targeting major transportation hubs like Volgograd to embarrass the Kremlin and discourage attendance at the Feb. 7-23 Olympics.

“The attack demonstrates the militants’ capability to strike at soft targets such as transport infrastructure outside of their usual area of operations in the North Caucasus,” he said in a note. “Although the very strict security measures which will be in place at the Sochi Games will make it difficult to undertake a successful attack against the main Olympic venues, public transport infrastructure in Sochi and the surrounding Krasnodar territory will face an elevated risk of attack.”

A city of 1 million northeast of Sochi, Volgograd is a hub with railway lines running in five directions across the country and numerous bus routes connecting it to the volatile Caucasus provinces.

Security checks on buses have remained largely symbolic and easily avoidable, making them the transport of choice for terrorists in the region. And tighter railway security isn’t always enough to prevent casualties. In Sunday’s attack, a suicide bomber detonated an explosive in front of the train station’s metal detectors, killing 17 people, including the attacker.

Security at Russia’s railway stations and airports was tightened after a male suicide bomber hit Moscow’s Domodedovo Airport in January 2011, killing 37 people and injuring more than 180. Two bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120.

Umarov, who had claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass demonstrations against Putin in the winter of 2011-12. He reversed that order in July, urging his men to “do their utmost to derail” the Sochi Olympics, which he described as “satanic dances on the bones of our ancestors.”

Aware of the threat, the Sochi organizers say they have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.

Anyone wanting to attend the games will have to buy a ticket online from the organizers and obtain a “spectator pass” for access. Doing so will require providing passport details that allow authorities to screen all visitors.

The security zone created around Sochi stretches about 60 miles along the Black Sea coast and up to 25 miles inland. Russian forces including special troops will patrol the forested mountains flanking the resort and use drones to keep a constant watch over Olympic facilities. Speedboats will patrol the coast and sophisticated sonar will be used to detect submarines.

Cars from outside the Olympic zone will be banned beginning a month before the Winter Games, and Sochi residents already are facing widespread identity checks.

The White House said the U.S. would welcome “closer cooperation” with Russia on security preparations for the safety of athletes, spectators and other participants at the Olympics after the two attacks.

“The U.S. government has offered our full support to the Russian government in security preparations for the Sochi Olympic Games,” White House spokesman Caitlin Hayden said.

Officials say that while Russia will be responsible for overall security at the games, U.S. personnel will be in the country in what President Barack Obama’s administration is calling “liaison roles.” The effort will be headed by the State Department’s Bureau of Diplomatic Security, which typically assists host nations during events where a large number of Americans will be present.Diplomatic security agents will work with Russian security and law-enforcement officials.

The State Department on Monday urged Americans planning to attend the Olympics to remain alert at all times.

“Major events such as the Olympic Games are an opportunity for thieves or for other folks who want to cause mischief,” State Department spokesman Marie Harf said. “Threats have been made against the Olympic Games, and acts of terrorism, including bombings, continue to occur in Russia.”

Some observers have warned that terrorists may attack softer targets in the vicinity of Sochi to sow panic.

“Even if they succeed in protecting Sochi, there could be a series of major attacks near Sochi,” Anatoly Yermolin, a veteran KGB officer, told Ekho Moskvy radio.

Alexei Filatov, another veteran of the Russian security forces, also predicted that terrorists will try to step up their attacks before the Olympics.

“The terrorist activities will increase as the Sochi Olympics get closer, and they will get increasingly close to the area,” Filatov wrote on his blog. “For those who order terror attacks, it serves as an opportunity to deal a blow to Russia on a global stage.”

Some Russian commentators also have suggested that terrorists could have planted sleeper agents in Sochi long before security was tightened. Others indicated that terror groups could have rigged some Olympic facilities with explosives during construction. Russian officials have denied that could happen, citing stringent security controls at Olympic construction sites.

Russian Olympic Committee chief Alexander Zhukov insisted Monday that there was no need to take any extra steps to secure Sochi in the wake of the Volgograd bombings because “everything necessary already has been done.”

Swedish hockey player Johan Franzen of the Detroit Red Wings still thinks more will be done.

“I’m sure after this, the security will be higher than they intended from the start,” he said.

U.S. figure skater Ross Miner recalled the 2011 world championships in Moscow as “by far the most intense security I’ve ever had at a competition,” with skaters going through metal detectors to enter the venue.

U.S. speed skaters, however, have different memories of Moscow. They arrived for a 2011 World Cup event the same day as the attack on Domodedovo Airport.

“It’s terrible we have to live in fear, but that’s just kind of how it is,” two-time gold and silver medalist Shani Davis said.

Meanwhile, a number of Olympic leaders and federations signaled their confidence in the host country.

“When we come to Sochi, it will be impossible for the terrorists to do anything,” Norwegian International Olympic Committee member Gerhard Heiberg said. “The village will be sealed off from the outside world. Security has been our priority No. 1 ever since Sochi got the Games.”

International Olympic Committee President Thomas Bach offered his condolences Monday in a letter to Putin and expressed full confidence that Russia would deliver “safe and secure games in Sochi.”

At the United Nations, the Security Council condemned the attacks as “heinous and cowardly acts,” and Secretary-General Ban Ki-moon spoke to Putin by telephone, offering his condolences and stressing the importance of international cooperation to fight terrorism, according to the U.N. press office. The White House also has condemned the attacks and offered condolences to those injured and the families of those killed.

Putin, who has made no public remarks since the first attack, ordered the tightening of security across the country after holding a series of meetings with government and security officials, according to the Kremlin. He dispatched the director of the Federal Security Service, Alexander Bortnikov, to Volgograd to oversee the investigation and enhanced security measures.

“I think we will be able to solve these crimes, particularly because we have some clues,” Bortnikov said after arriving there, without elaborating on the evidence investigators were pursuing. He said that additional security had been deployed in public places, including the city’s transportation and energy facilities.

Vladimir Markin, the spokesman for Russia’s main investigative agency, the Investigative Committee, said Monday’s explosion involved a bomb similar to the one used Sunday, indicating that the two attacks were linked. At least 12 people have been detained by security officials.

Markin said a suicide attacker was responsible for Monday’s bus explosion that killed at least 14 people. It was not clear if authorities counted the bomber in their death toll.

In October, a suicide bombing on a bus in Volgograd killed six people.

Information for this article was contributed by Vladimir Isachenkov, Eddie Pells, Julie Pace, Sagar Meghani, Deb Riechmann, Paul Newberry, Howard Fendrich, Rachel Cohen, Larry Lage, Stephen Wilson, Stephen Douglas, Karl Ritter, Eric Willemsen, Jim Heintz and Alexandra Olson of The Associated Press; by Steven Lee Myers, David Herszenhorn, Nikolay Khalip and Viktor Klimenko of The New York Times.

Front Section, Pages 1 on 12/31/2013