Russian President Vladimir Putin says all possible causes, including terrorism, are being considered in the investigation into a blast that killed at least nine people and left more than 50 injured, according to Russian media.

Members of the Emergency services stand next to a helicopter outside Tekhnologicheskiy institut metro station in St. Petersburg, Russia April 3, 2017.  [Photo/Agencies]

Putin was in St Petersburg around the time the blast happened on a train between Sennaya Ploshchad and the Institute of Technology stations. He said he had conferred with security forces.

"Security forces are working and doing what they can to ascertain what happened and make a complete assessment of the situation," he was quoted by the BBC as saying.

The Proscutor General’s office in St Petersburg described it as a terror attack.

Initially, Russian media reported there had been two blasts but officials are now saying there was one explosion on a train traveling between the two stations.

They also said nine, and not 10 passengers had been killed, as was initially reported.

The Associated Press said a device had been found and defused at another St Petersburg Metro station.

Pictures posted on the internet showed a train with twisted and buckled doors, and people lying on the station platform, apparently seriously injured.

All of St Petersburg’s metro stations were closed and passengers had been evacuated, the BBC said.

In Moscow, subway authorities were taking extra security precautions and offered help to their colleaguesin St Petersburg.

The St Petersburg metro consists of five lines and carries about two million passengers a day.