Donald Trump has offered his condolences to the victims of the St Petersburg metro explosion.
According to the White House the US President called his Russian counterpart Vladimir Putin on Monday to offer "the full support of the United States government in responding to the attack and bringing those responsible to justice."
Russia President Vladimir Putin has laid a floral tribute at a impromptu memorial to the victims of a bomb blast on the St Petersburg metro station.
Mr Putin happened to be in Russia's second-largest city for a meeting with his Belarusian counterpart Alexander Lukashenko when the explosion took place.
Authorities in Russia have initially identified the suspect behind the St Petersburg metro blast.
They believe a young person, believed to a 23-year-old man from Central Asia and linked with radical Islamists, was a suicide bomber and carried an explosive device onto the train in a rucksack.
Police believe the same man, not a second suspect, planted a bomb at another subway station that was defused before it went off.
Floral tributes and candles have been left by mourners in memory of the victims of the metro bombing in St Petersburg.
The tributes were left at an improvised memorial on Moskovsky Prospekt Street near the entrance to Tekhnologichesky Institute station.
The death toll in the subway bombing in St. Petersburg has reached 11, Russia's top anti-terror agency has said.
The bombing of a subway train in Russia has drawn condemnation from world leaders.Read the full story ›
A blast has ripped through an underground train in Russia's second-largest city on Monday.Read the full story ›
Russia Prime Minister Dmitry Medvedev has described the blast on the St Petersburg metro station as a "terrorist attack".
In a post on his twitter account, Mr Medvedev sent his "most sincere condolences to the families and relatives of the victims of the explosion" and said the injured would be given "all necessary assistance".
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Search warrants have been issued for two people over the St Petersburg metro blast, according to Interfax news agency.
Officials in Paris have increased security at public transport locations "as a precautionary measure" following the explosion in a St Petersburg underground tunnel on Monday.
In a statement, the interior ministry said: "Following the events in the St. Petersburg metro, and as a precautionary measure, Interior Minister Matthias Fekl has decided to redeploy security means in public transport across the Paris region.
"Amid an extremely high terrorism threat, the government continues to take measures to protect the French people."