The blasts on the St Petersburg Metro could be terror attacks, Vladimir Putin has said.
The Russian government is trying to establish the cause of the blasts and all causes are being considered, including terrorism, he said. A terror attack would be considered "first of all", he said in a statement.
“The causes of this event have not been determined yet, so it’s too early to talk about [possible causes]," Mr Putin said. "The investigation will show. Certainly, we will consider all variants, common, criminal, first of all, of a terrorist nature."
Mr Putin said that the cause of the incident still isn't clear, but that he had spoken to the heads of the Russian security services.
He offered his condolences to the families of those killed in the attack.
Mr Putin was visiting the city on an unrelated trip on the day of the attacks. He was due to meet with the Belarusian president later on.
As many as 50 people are thought to have been injured in the attack, with officials saying that 10 people were dead after the explosion.
Russia's National Anti-Terrorist Committee, which said "several" people were killed and injured, said an unidentified explosive device went off on a train that was travelling between two stations.
The agency that runs the subway said several stations in the northern Russian city were closed and that an evacuation was under way.
Social media users posted photographs and video from a subway station in the city centre, showing people lying on the floor and a train with a mangled door nearby.
Frantic commuters reached into doors and windows, trying to see if anyone was there and shouting: "Call an ambulance!"
