"Deeply saddened by the loss of lives in the blasts at St. Petersburg metro. Heartfelt condolences to the families of the victims," he tweeted.
At least 10 people were killed and 50 were injured today in the explosions in St Petersburg.
President Vladimir Putin, who was visiting the city on an unrelated trip, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause.
Russia's National Anti-Terrorist Committee said an unidentified explosive device went off on a train that was traveling between two stations.