Police are hunting for clues after a bomb blast tore through a subway train in St Petersburg, killing at least 11 people and wounding more than 40.

There was no immediate claim of responsibility for the attack, which came while President Vladimir Putin was visiting the city, his hometown.

The explosion occurred in mid-afternoon on Monday as the train travelled between stations in the centre of the city.

News reports initially said police were searching for two suspects, but the Interfax news agency later cited unspecified sources as saying police now suspect the blast was the work of a suicide bomber linked to "radical Islamists".

The National Anti-Terrorism Committee said it was looking for the "perpetrators and organisers of the terror attack".

Amateur video broadcast by Russian TV showed people lying on the platform of the Technological Institute station, and others bleeding and weeping just after the train pulled in with a huge hole ripped in the side of one of the carriages.

"When I turned back and looked, there was a huge number of people lying there. There were dead bodies," said Natalya Kirrillova, a witness.

"It was scary. And when we left, they took out several people covered in blood. [There was] a woman, her whole face was a massive wound."

Second device

Within two hours of the blast, authorities had found and deactivated another bomb at another busy station, Vosstaniya Square, the anti-terror agency said. That station is a major transfer point for passengers on two lines and serves the railway station to Moscow.

Interfax cited an unidentified law enforcement official saying that investigators think the suspected suicide bomber left the bomb at the Vosstaniya Square station before blowing himself up on the train.

Attacks in Russia 2013: Suicide bombers targeted a train station and a trolley bus in Volgograd. At least 34 killed. 2011: 37 people died in a suicide bombing at one of Moscow's international airports. Chechen fighters claimed responsibility. 2010: Suicide bombers detonated bombs in the Moscow metro, killing 38. A Chechen group claimed the attack. 2009: At least 28 killed when a high-speed rail link between Moscow and St Petersburg was hit by a suicide attack. Chechen fighters were held responsible.

Putin, who was meeting the president of Belarus at the Constantine Palace on the city's outskirts, offered condolences on national television.

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

Three days of mourning began on Tuesday.

The UN Security Council condemned "in the strongest terms the barbaric and cowardly terrorist attack", adding that the "perpetrators, organisers, financiers and sponsors" should be brought to justice.

US President Donald Trump said it was "absolutely a terrible thing". White House Press Secretary Sean Spicer said the US was prepared to offer assistance to Russia.

Lebanon's Hezbollah group, which is backing Syrian President Bashar al-Assad's forces along with Russia, said the incident was the type of "terrorism" Russia was fighting in Syria.

In the past two decades, Russian trains and planes have been frequent targets of attacks, most of them connected to the insurgency in Chechnya and other Caucasus republics in the southern part of the country.

Two female suicide bombers killed 40 people and wounded more than 100 in the Moscow metro in March 2010. Chechen rebel leader Doku Umarov claimed responsibility for the attack, warning Russian leaders that "the war is coming to their cities".

A Moscow-to-St Petersburg train was bombed in November 2009, in an attack that left 28 dead and 100 injured. Umarov's group said he ordered this attack.