Ten people were killed and many more injured Monday when an explosion rocked a subway train in the Russian city of St. Petersburg, Tass and other Russian news agencies reported.

Several metro stations were shut down following the blast, Tass said. Photos and video posted on social media from the scene showed a smoke-filled platform, a damaged subway train and people lying on the ground.

"So far, we say it was an unidentified explosive device," Andrei Przhezdomsky, a spokesman for the National Anti-Terrorism Committee told the Rossiya-24 TV. "Investigators and the Federal Security Service’s bomb specialists are to establish the exact cause of this explosion."

Russian President Vladimir Putin, who was in Russia's second-largest city at the time, offered condolences to families and loved ones of those killed.

“The causes of this event have not been determined yet," Putin said. "Certainly, we will consider all possibilities: common, criminal, but first of all of a terrorist nature.”

Andrei Kibitov, spokesman for the St. Petersburg governor, told Russian TV that 10 people were killed and 50 wounded in the blast, the Associated Press reported. Russia’s National Anti-Terrorism Committee ordered tightened security at all critical transport facilities, AP said.

Interfax said local prosecutors had turned over the probe to the federal Investigative Committee.

St. Petersburg, a city of about 5 million people on the Baltic Sea in western Russia, was founded by Peter the Great and served as Russia's capital for two centuries. The city remains a cultural center. Tass said the explosion took place near the Sennaya Ploshchad station, an area of the city sometimes known as Hay Square for its history as a trading center for hay, straw and wood.