Russian security agencies have found and defused a second explosive device on the St Petersburg metro after an explosion killed at least nine people.

The device was left in a briefcase, the Interfax news agency reports.

Andrei Przhezdomsky, a spokesman for Russia's top anti-terror agency, said the improvised explosive was found at the Ploshchad Vosstaniya station.

10 killed, 50 hurt in St Petersburg subway blast

An unidentified explosive device went off at 2:20pm on a train leaving the Technology Institute station and heading to the Sennaya Ploshchad station, Russia's National Anti-Terrorist Committee said.

The blast came as Russian President Vladimir Putin was visiting the city, his hometown. He said investigators are looking into whether the explosion was a terror attack.

News agency Interfax cited a source saying the blast was caused by a bomb filled with shrapnel.

St. Petersburg metro blast Show all 5 1 /5 St. Petersburg metro blast St. Petersburg metro blast An injured person is helped by emergency services outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Emergency services work outside Sennaya Square metro station in St Petersburg Reuters St. Petersburg metro blast An injured person is helped by emergency services outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Emergency services direct pedestrians outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg Reuters St. Petersburg metro blast Twitter/ varlamov

Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station near the centre of the former Russian imperial capital. A helicopter hovered overhead as crowds gathered.