Russian authorities said at least 10 people were killed in explosions in two train carriages at metro stations in St. Petersburg on Monday.
Russian President Vladimir Putin, who is in St. Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the blasts was not clear and efforts were underway to find out.
He said he was considering all possibilities including terrorism.
A Reuters witness saw eight ambulances near the Sennaya Ploshchad metro station.
Three stations have been closed, local media outlet Fontanka reported.
Video showed injured people lying bleeding on a platform, some being treated by emergency services. Others ran away from the platform amid clouds of smoke.
Russia has been the target of attacks by Chechen militants in past years. Chechen rebel leaders have frequently threatened further attacks.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
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