Blast in St. Petersburg metro station kills 9: authorities

Interfax news agency quoted an unnamed source as saying the blast, which occurred when the train was between two stations, was caused by a bomb filled with shrapnel.

An injured person stands outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg. REUTERS/Anton Vaganov

President Vladimir Putin, who was in the city for a meeting with Belarus's leader, said he was considering all possible causes for the blast, including terrorism and was consulting with security services.

Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station. A helicopter hovered overhead as crowds gathered to observe rescue operations.

Police officers are seen outside Ploschad Revolyutsii (Revolution Square) metro station in Moscow, Russia, April 3, 2017. REUTERS/Sergei Karpukhin

"I appeal to you citizens of St. Petersburg and guests of our city to be alert, attentive and cautious and to behave in a responsible matter in light of events," St Petersburg Governor Georgy Poltavchenko said in an address.

General view of emergency services attending the scene outside Sennaya Ploshchad metro station, following explosions in two train carriages in St. Petersburg, Russia April 3, 2017. REUTERS/Anton Vaganov

An attack on St Petersburg, Russia's old imperial capital, would have some symbolic force for any militant group, especially Islamic State or Chechen secessionist rebels. Attacks in the past have largely concentrated on Moscow, including an attack on an airport, a theater and in 2010 a metro train.

Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.

A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage. Russian TV said many had suffered lacerations from glass shards and metal.

Russia has been the target of attacks by separatist Islamist Chechen militants in past years. Islamic State, which has drawn recruits from the ranks of Chechen rebels, has also threatened attacks across Russia in retaliation for Russian military intervention in Syria.

The Russian air force and special forces have been supporting President Bashar al-Assad in fighting rebel groups and Islamic State fighters now being driven out of their Syrian strongholds.