The US President expressed his condolences to the victims’ families in a phone call with the Russian leader.
He added: “The presidents noted that terrorism is an evil that must be jointly fought with.”
Russian leader Vladimir Putin said in a statement: “The causes are not clear, so it is premature to speak of this. The investigation will reveal.
Putin was in the city at the time of the blast and visited the scene in the evening to lay flowers in a show of respect to the victims of the attack.
Officials now believe a suicide bomber was behind the attack. Investigators believe their suspect had links to radical Islam.
Injured people were pictured lying bleeding on a platform as passengers and emergency services rushed to help them amid a flurry of smoke at 2.30pm local time (12.30pm GMT).
Passengers were immediately evacuated from the stations, the metro company said.The driver of the metro has been hailed for driving the train forward as it almost certainly helped save lives and allowed people to be rescued quickly, senior investigator Svetlana Petrenko said.No terror groups have claimed responsibility for the blast.