The Obama administration on Monday condemned two suicide bombings in Russia which killed dozens and said it hoped for “closer cooperation” between Washington and Moscow to prevent terror attacks during the upcoming Winter Olympics in Sochi.

“The United States condemns the terrorist attacks that struck the Russian city of Volgograd and sends deepest condolences to the families of the victims with hopes for the rapid healing of those wounded,” said National Security Council spokesperson Caitlin Hayden. “The United States stands in solidarity with the Russian people against terrorism.”

Hayden said that the administration was offering its assistance to Moscow ahead of the Olympics.

“The U.S. government has 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,” she said.

Russian President Vladimir Putin ordered tighter security across Russia on Monday after a suicide bomber targeted a trolley car in the city of Volgograd, killing at least 15 people and wounding dozens, according to reports.

It was the second attack in the central Russian city in as many days. On Sunday, a suicide bomber detonated an explosive at the main train station.

Russian officials said that no groups had claimed responsibility for the attacks, but that the two explosive devices appeared to be similar.

Russia is set to host the Winter Games in February, raising fears that terrorists could strike during the event.

Russia has long faced violence in its Caucasus region from violent Islamic groups seeking greater autonomy from Moscow. In 2010, suicide bombers targeted the Moscow Metro, killing dozens in the capital city.

Tamerlan Tsarnaev, suspected of carrying out the Boston Marathon bombing, is believed to have met with radical Islamic groups during a visit to the Russian region of Dagestan, months before the deadly terror attack.

The Sochi Olympics also sparked controversy after the Obama administration criticized Russian lawmakers for passing anti-gay rights legislation.

While President Obama refused calls from some groups to boycott the games, his decision to name openly gay athletes to the presidential delegation to the games was seen as a snub to Putin. The president and first lady and other top administration officials will not be attending the games.