President Barack Obama has condemned the rush hour bombings in the Moscow metro, offering his condolences to the Russian people.

In a statement issued by the White House early Monday, the president said the American people stand united with the people of Russia in opposition to violent extremism and "heinous" terrorist attacks.

The blasts came about 30 minutes apart during the morning rush hour, killing at least 37 people and wounding 102.

The head of Russia's main security agency said a preliminary investigation places the blame on rebels from the restive Caucasus region that includes Chechnya, where separatists have fought Russian forces since the mid-1990s.