About 10 people have been killed in an explosion between two underground stations in St Petersburg.
The head of Russia’s National Anti-Terrorist Committee said the blast hit the train between Sennaya Ploshchad and Tekhnologichesky Institut stations.
The committee said an explosive device was later found and made safe at another station, Ploshchad Vosstaniya.
President Vladimir Putin said all causes, including terrorism, were being investigated.
Initial reports suggested there had been two explosions, one at Sennaya Ploshchad and Tekhnologichesky Institut stations.
A spokesman for St Petersburg’s governor said at least 10 people had been killed and 50 injured. But minutes later, Russian National Anti-Terrorist Committee said the death toll was nine, with 20 hurt.
Andrei Przhezdomsky, the head of the committee, said the explosion at 14:40 local time (12:40 BST) was caused by “an unidentified explosive device” but that the exact cause had yet to be determined.
President Putin was in St Petersburg earlier on Monday but is now outside the city, his spokesman Dmitry Peskov said.
“I have already spoken to the head of our special services, they are working to ascertain the cause,” he said, at a meeting with Belarus President Alexander Lukashenko.
The entire St Petersburg underground network has now been shut down, and Moscow metro officials said they were introducing extra security measures as a result.
St Petersburg’s metro system is the 19th busiest in the world, with more than two million passengers every day. It has not suffered attacks before.
However, several transport hubs in Russia have been attacked. In 2010 at least 38 people died in a double suicide bombing on the Moscow metro.
In 2009, a bomb exploded on a high speed train travelling between Moscow and St Petersburg, killing 27 and injuring another 130.
Both attacks were claimed by Islamist groups.
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