At least 10 people were killed in explosions in two train carriages at metro stations in St. Petersburg on Monday, Russian authorities said.

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.

There are 50 injured in St Petersburg - Interfax cites governor's press office.

At least nine people were killed and 20 were injured when an explosion tore through a train carriage in the St.Petersburg metro system on Monday, the Russian National Anti-Terrorist Committee said.

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.

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.

"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.

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 theatre 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.

ALL STATIONS CLOSED

St. Petersburg emergency services at first said that there had been two explosions. But a source in the emergency services later said that there had been only one but that the explosion had occurred in a tunnel between stations.

The blast occurred at 2.40 p.m., well shy of the evening rush hour.

Authorities closed all St. Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there.

Russia has been on particular alert against Chechen rebels returning from Syria and wary of any attempts to resume attacks that dogged the country several years ago.

At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.

Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theatre to end another hostage-taking.

Putin, as prime minister, launched a 1999 campaign to crush a separatist government in the Muslim southern region of Chechnya, and as president continued a hard line in suppressing rebellion.