At least 10 people were killed in an explosion in the metro system of St Petersburg, Russia, today, according to preliminary information, Russian news agency Tass said.

The explosion was inside a metro train at the Sennaya Square station in St. Petersburg, Tass said quoting emergency services.

According to a source, an unidentified device exploded in a train car.

"According to information available at this point ten people died. Others were injured," a source said adding that the number of casualties was still to be specified.

Rescue workers and law enforcement personnel are working at the scene. Evacuation is under way at the blast-hit subway station. Seven metro stations in the city have been closed following the blast, Tass said.  

A spokesman for the regional department of Russia’s Emergencies Ministry said it was checking reports on the smoke in the metro. "There is a complaint about the smoke, the staff members are going to the scene," he said.

Another agency, RIA Novosti, said there appeared to have been blasts at two metro stations.