An explosion on a subway in St. Petersburg has killed at least ten people and injured scores more in a terrorist attack.

The head of Russia’s National Anti-Terrorist Committee said the blast hit the train between Sennaya Ploschad and Tekhnologichesky Institut stations.

Images posted on social media showed a badly mangled carriage, with a number of casualties nearby.

President Vladimir Putin said all causes, including terrorism, were being investigated.

Initial reports suggested there had been two explosions, one at each of the two stations.

At least 50 people have been reported injured, and a number of children were among those hurt.

Andrei Przhezdomsky, the head of the Russian National Anti-Terrorist Committee, said it was caused by “an unidentified explosive device” but that the exact cause had yet to be determined.

However, a spokesman for Russia’s prosecutor general said the explosion was “a terrorist act”.

Eyewitness video shows immediate aftermath of blast at St. Petersburg metro station https://t.co/L2WMWTljKN pic.twitter.com/RpO9SYu2wD

— CNN (@CNN) April 3, 2017

Russian General Prosecutors Office calls St Petersburg blast a terrorist attack. All 62 stations on lock-down. Increased security in Moscow

— John Sparks (@sparkomat) April 3, 2017

10 people have been killed & 50 are injured after deadly blasts rock St. Petersburg subway, Russian media reports https://t.co/sOwLJ2XsZk pic.twitter.com/tWNlBJZmCT

— CBS News (@CBSNews) April 3, 2017

Russian official says there was one blast on St Petersburg metro, not two, with up to 10 people killed https://t.co/PwbDHdxDCv pic.twitter.com/faOB772Wm2

— BBC Breaking News (@BBCBreaking) April 3, 2017

Russian authorities are saying that a second bomb did not go off, and was deactivated by authorities.

As of now there are no confirmed claims of responsibility for the attacks.