At least 10 people have been killed and another 50 injured in what is being reported as an unidentified explosion on St. Petersburg’s subway system in Russia on Monday afternoon. The bomb blast was said to have been detonated on a train that was traveling between two stations – Sennaya Ploshchad and Tekhnologicheky Institut – at around 2.30pm local time.

European media outlets had blanketed coverage of the event shortly after it occurred with the BBC, Sky News and major UK news outlets such as The Guardian, The Telegraph and tabloid the Daily Mail all leading with the story.

Russia’s National Anti-Terrorism Committee confirmed that at least 10 people were killed while Russia’s Interfax news agency quoted an unnamed law enforcement source as saying the explosion was caused by a bomb filled with schrapnel.

President Vladimir Putin was in St. Petersburg at the time for a meeting with Belarussian leader Alexander Lukashenko and expressed his condolences to families of the victims. He said it was “too early to say” what or who was behind the explosion but suggested it could be “criminal or terrorist.” He said he was speaking to heads of security services who were currently trying to figure out the cause.

It has been reported that all stations on the St. Petersburg Metro have now been closed and the St Petersburg underground railway issued a statement saying that evacuations were ongoing.

Russia has suffered frequent terror attacks from fundamental Islamist groups based in North Caucaus and has been on alert against Chechen rebels returning from Syria. In 2010, 40 people were killed when two suicide bombers detonated explosions on Moscow’s packed metro trains while at least 35 people were killed and 130 injured in a suicide bombing at Moscow’s Domodedovo aiport in 2011. In 2009, 26 people were killed and some 100 injured when a high-speed train from Moscow to St. Petersburg was bombed.