(CNN)At least 10 people have been killed in twin blasts on a metro in the Russian city of St. Petersburg, state-run TASS reported.

The explosions took place on a subway car at the station.

Kremlin spokesman Dmitry Peskov said President Vladimir Putin was aware of the reports of the explosions, state-run Russia 24 reported.

Putin was in St. Petersburg speaking at an event earlier Monday but it was not clear if he was still in the city.

A source earlier told TASS the explosions took place at the Sennaya Ploshchad station.

St. Petersburg is Russia's second city.

Developing story – more to come