The likely suspect in a deadly blast in the Russian city of St Petersburg was a Kyrgyz-born Russian citizen, Kyrgyzstan's security service said yesterday.

The blast in a train carriage between two underground stations killed 11 people and wounded 45. A Russian law enforcement source said the attack was carried out by a suspected suicide bomber with ties to radical Islamists, according to Russia's Interfax news agency.

A spokesman for Kyrgyzstan's GKNB security service said it had received an official request from Russian security services to identify the suspect. The spokesman declined to name the suspect.

Kyrgyzstan, a predominantly Muslim Central Asian nation of six million, is Russia's close political ally and hosts a Russian military airbase.

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Russian President Vladimir Putin, who was in the city when the blast struck, visited the scene of the explosion yesterday and laid a bunch of red flowers at a makeshift shrine to the victims.

Witnesses said they saw passengers who were bloodied and burned spilling out of the train, the door of which was buckled by the force of the explosion, and lying on a platform while smoke filled the station.

Russia has suffered bomb attacks by Islamist rebels from Russia's North Caucasus region in the past. The rebellion there has been largely crushed, but Russia's military intervention in Syria has made it a potential target for Isis (Islamic State) attacks, security experts say.

There was no immediate claim of responsibility but officials said they were treating the blast as an act of terrorism.

The news agency, quoting an unidentified law enforcement source, said that human remains examined at the scene suggested that the blast had been carried out by a suicide bomber. It said the police had identified a suspect with links to radical Islamist groups banned in Russia.

If it is confirmed that the bomb was carried out by radical Islamists, the Kremlin is likely to argue the attack underlines the importance of its campaign in Syria, where it is backing President Bashar al-Assad in a fight against Islamist militants.

Russian President Vladimir Putin lays flowers at a place near the Tekhnologichesky Institut subway station in St.Petersburg, Russia. Photo / AP
Russian President Vladimir Putin lays flowers at a place near the Tekhnologichesky Institut subway station in St.Petersburg, Russia. Photo / AP

However, some sections of Russian society could see the metro bombing as proof that Putin's decision to intervene in Syria has again made Russian civilians into targets.

United States President Donald Trump offered his condolences to Putin in a phone call, the White House said in a statement, offering the support of the US Government. The two leaders "agreed that terrorism must be decisively and quickly defeated", the statement said.

China also condemned the attack, with Foreign Minister Wang Yi offering condolences and support in a statement that said "China resolutely opposed all types of terrorism".

The blast raised security fears beyond Russian frontiers. France announced additional security measures in Paris.

Soon after the blast happened, ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad station. One helicopter hovered overhead and then landed on a broad avenue to take away a wounded passenger.

"I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people's insides on their clothes, bloody faces," St Petersburg resident Leonid Chaika told Reuters by phone. "Many were crying," said Chaika, who was at the station where the blast happened.

People carrie a subway blast victim into an ambulance after explosion at Tekhnologichesky Institut subway station in St.Petersburg, Russia. Photo / AP
People carrie a subway blast victim into an ambulance after explosion at Tekhnologichesky Institut subway station in St.Petersburg, Russia. Photo / AP

The National Anti-Terrorist Committee said an explosive device had been found at another station, hidden in a fire extinguisher, but had been defused.

Video from the scene showed wounded people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.

A huge hole was blasted in the side of a carriage and the door blown off, with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage after the train had pulled into the station.

Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion amplified by the confines of the carriage and the tunnel.

Anna Sventik, a St Petersburg resident, said she was travelling on a metro train that passed through the same station moments after the blast.

"Our train slowed down a bit, and one woman started having hysterics when she saw the people lying on the platform, blackened, in some places with no clothes, burned," she told Reuters. "It was very scary."

Authorities closed all St Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there.

Russia has been on particular alert against Russian-speaking rebels returning from Syria, where they have fought alongside Isis, and is wary of any attempts to resume attacks that dogged the country several years ago.

- Reuters