Updated April 04, 2017 01:05:43

At least 10 people have been killed in an explosion on a train carriage in St Petersburg's underground metro, Russian authorities say.

Fifty people were injured in the blast, Andrey Kibitov, the head of the city governor's press service said on Twitter.

Russian President Vladimir Putin, who is in St Petersburg for a meeting with Belarussian leader Alexander Lukashenko, said the cause of the explosion was not clear.

"I have already spoken to the head of our special services, they are working to ascertain the cause [of the blasts]," Mr Putin said at a meeting with Mr Lukashenko.

"The causes are not clear, it's too early. We will look at all possible causes, terrorism as well as common crime."

Russia's National Anti-Terrorist Committee said another explosive device had been found at a different metro station, but it had been made safe.

There was no immediate claim of responsibility for the blast.

The explosion happened at about 2:40pm (local time), well ahead of the evening rush hour, on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, the National Anti-Terrorist Committee said.

Photo: The train hit by an explosion at the Tekhnologichesky Institut subway station in St Petersburg. (AP)

Ambulances and fire engines descended on Sennaya Ploshchad near the centre of the former Russian imperial capital. A helicopter hovered overhead as crowds gathered.

"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.

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.

Security stepped up across transport network

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

The National Anti-Terrorism Committee announced security would be tightened at all critical transport facilities following the blast.

Video showed injured people on a platform, some being treated by emergency services. Others ran away from the platform amid clouds of smoke.

A train could be seen stopped at a platform with mangled doors.

St Petersburg, Russia's second-largest city with over 5 million residents, is the country's most popular tourist destination.

The two stations that were the site of the blast are some of the subway's busiest.

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.

Photo: Video showed emergency services at the scene. (AP)

Reuters/AP

Topics: unrest-conflict-and-war, russian-federation

First posted April 03, 2017 22:29:09