Officials may have caught the organiser of the attacks on the St Petersburg metro on CCTV, according to reports.

Surveillance cameras captured pictures of the supposed organiser of the blast, Interfax reported. "Images of the suspected organiser of the metro blast were captured on metro station cameras," the source was reported to have said.

The explosive device might have been left in a briefcase on a train carriage, the same source told the Russian news organisation.

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

At least 10 people died and around 50 were injured in a huge blast that hit the city's public transport system.

Vladimir Putin, who was in the city at the time of the attack, said that officials hadn't yet found a motive but that they would look at a terror attack "first of all". Isis supporters have been praising the attack, but neither that group or any other has publicly claimed responsibility for it.

The unidentified explosive device went off at 2:20 p.m. on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, the agency said.

Social media users posted photographs and video from the Technology Institute subway station, showing injured people lying on the floor outside a train with a mangled door. Frantic commuters were reaching into doors and windows, trying to see if anyone was there, and shouting "Call an ambulance!"