A frantic manhunt is underway for two suspected terrorists after a nail bomb exploded on the St Petersburg metro network, killing 10 people and injuring 40 more.

Russian media outlets have released a CCTV picture of one bearded suspect, who was seen wearing a long, black top and a black hat. He is thought to have detonated the device, contained in a briefcase, after changing carriages.

The bomb went off as Vladimir Putin, the Russian President, was visiting the country’s second city and his home town. Speaking on television from Constantine Palace, he offered his condolences to the families of those killed.

"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," said Mr Putin.

Andrei Przhezdomsky, head of the National Anti-Terrorist Committee, said the explosion was caused by "an unidentified explosive device".

Dmitry Medvedev, the Prime Minister, later said in a Facebook post that the explosion was a "terrorist attack".

Boris Johnson, the Foreign Secretary, said he was horrified by news of the explosion. “My sympathies are with the victims and their families,” he tweeted.

In response, the French Government said it was upping security at transport hubs in Paris.

Veronika Skvortsova, Russia’s Health Minister, said 10 people had died from the blast - seven at the scene, one in an ambulance and two in hospital - while City health authorities confirmed 43 others had been taken to hospital. Three are thought to be children.

Media reports suggested the death toll could rise.

The terrifying blast hit an underground train around 2.20pm as it travelled between the stations of Sennaya Ploshchad and Tekhnologichesky Institut in the centre of the city.

Another bomb, reportedly disguised as a fire extinguisher, was later found at a third station nearby, Vosstaniya Square, and made safe. A second person is now being hunted in relation to this device.

After the blast, TV pictures showed the train carriage doors buckled and blown open. The subway was filled with smoke. Bodies lay on the platform, which was strewn with debris. Above ground paramedics tended the injured before they were transferred to hospital.

This station is a major transfer point for passengers on two lines and serves the overground railway station from which most trains to Moscow depart.

Russian law enforcement agencies confirmed the Vosstaniya Square device was rigged with shrapnel and the Interfax news agency said it contained up to 2.2lbs of explosives.

The blast lead to the entire St Petersburg underground system, which serves some two million passengers a day, being shut down and evacuated.

The metro train driver, who chose to continue on to the next station after the blast, was praised by the authorities because his actions helped evacuation efforts and reduced the danger that passengers would die by trying to walk along the subway's electrified tracks.

Witnesses said the blast spread panic among passengers, who ran towards the exits of the station, which lies 130 feet underground.

"Everything was covered in smoke, there were a lot of firefighters," said student Maria Smirnova, who was travelling on a train behind the one hit by the blast.

"Firefighters shouted at us to run for the exit and everyone ran. Everyone was panicking," she added.

Another witness explained: “People were bleeding, their hair burned. We were told to move to the exit…People just fled. My girlfriend was in the next car that exploded...When she came out, she saw people were mutilated.”

There was no immediate claim of responsibility for the attack but Russian trains and planes have been targeted repeatedly by Islamic militants, mostly connected to the insurgency in Chechnya and other Caucasus republics.

The last confirmed attack was in October 2015 when so-called Islamic State militants downed a Russian airliner heading from an Egyptian resort, killing all 224 people on board.

The December 25 crash of a Russian plane carrying Red Army choir members near the southern city of Sochi is widely believed to have been due to a bomb but no official cause has been stated for the crash that killed 92 people.

Russian transport facilities have previously been the target of terror attacks.

In March 2010 suicide bombers in the Moscow subway killed 40 people and wounded more than 100 people.

Doku Umarov, the Chechen rebel leader, claimed responsibility for the attack undertaken by two women. He warned Russian leaders: “The war is coming to your cities."

A year earlier, a Moscow-to-St Petersburg train was bombed in an attack, which left 26 people dead and some 100 injured. Umarov's group said their leader had ordered the attack.

Russian airports have also been hit by attacks.

In January 2011, a suicide bomber blew himself up at Moscow's Domodedovo Airport, killing 37 people and wounding 180. In August 2004, the same airport saw Islamic suicide bombers board two planes and bring them down. A total of 90 people were killed.