MOSCOW (CNN) -- Ten people have been killed in an explosion on the metro in the Russian city of St. Petersburg, state-run media TASS reported.
Around 50 others were injured, the St. Petersburg governor's office said, in the incident that has forced the shutdown of the entire metro system.
The explosion took place on a subway car at the Sennaya Ploshchad station.
Images of the car shows the facade had been ripped off, while others showed passengers running from the site at the station filled with smoke.
"According to the very first preliminary information, about 10 people were killed in the explosion," a source told TASS, noting the exact number of victims was still being established.
"At the moment there are about 50 injured, doctors are working with them. The number of ambulances there at the moment is 17, it will increase," said the governor's press secretary, Andrei Kibitov.
Russian President Vladimir Putin expressed his condolences to the victims and is talking to the FSB security services about the investigation, according to state media.
He said he was not ruling out any causes behind the explosion.
"The reasons for the explosion are unknown, so it's too early to talk about it. The investigation will show what happened," Putin said, beginning a meeting with Belarussian President Alexander Lukashenko.
"Naturally, we always consider all options -- both domestic and criminal, primarily incidents of a terrorist nature."
Kremlin spokesman Dmitry Peskov said Putin had been in St. Petersburg speaking at an event earlier Monday.
St. Petersburg is Russia's second-biggest city.