WASHINGTON – The U.S. government is concerned Islamist militants may be preparing attacks aimed at disrupting the Winter Olympic Games in Sochi in February and is offering closer cooperation on security with Russia despite strains earlier this year.

Two bombings in the Russian city of Volgograd in the past two days - one at the city's central railway station and another on a bus - killed dozens of people and raised anxieties about the safety of the Olympics.

One militant group issued explicit direct threats to disrupt the Olympics, a State Department official said. Other officials said that regions near Sochi were among the areas of Russia currently most prone to Islamic militancy and other unrest.

"There are clearly sensitivities in our relationship with Moscow but enhancing Olympic security and counterterrorism efforts more broadly are areas of clear mutual interest," one U.S. official said.

"The Volgograd bombings underscore the threat and the need to work hand in hand with Russia in order to ensure the protection of U.S. citizens participating in and attending the games in Sochi," the official said.

U.S. security officials said the government was not surprised by the Volgograd bombings and had anticipated that such attacks might well occur in the run-up to the games.

The officials said U.S. and Russian authorities have engaged in extensive contacts regarding security preparations for the Olympics. The United States is expected to share with Russia information it might collect about possible threats to the games.

"We're taking lots of security precautions" related to the Winter Games, a U.S. State Department official said on Monday.

National Security Council spokeswoman Caitlin Hayden condemned the Volgograd attacks, which were blamed on suicide bombers.

She said the U.S. government had "offered our full support to the Russian government in security preparations for the Sochi Olympic Games, and we would welcome the opportunity for closer cooperation for the safety of the athletes, spectators, and other participants."

The State Department is expected to caution U.S. travelers on Monday about possible bombings and hostage takings in Russia, particularly in the North Caucasus region, which is less than 100 miles (160 km) from Sochi.

The U.S. offer for closer cooperation with Russia follows two issues earlier this year that raised tensions between U.S. and Russian security agencies: the involvement of two Chechen brothers in the Boston Marathon bombing and Russia's granting temporary asylum to former U.S. National Security Agency contractor Edward Snowden.

Nonetheless, in November, Matthew Olsen, director of the government's U.S. National Counter-terrorism Center, said his agency was "coordinating and integrating the intelligence community's support ... to the Winter Olympics in Sochi."

Olsen told a Senate committee he had visited Sochi a few days earlier and met Russian intelligence and security officials to discuss the games.