Isis supporters are cheering the attack in St Petersburg that killed at least 10 people.
While the motivations and cause of the attacks still aren't clear, Vladimir Putin has said that Russia is considering a terror attack "first of all".
Isis supporters are cheering what they claim is a terror attack, and sharing images of people caught up in and killed by the blasts.
The attacks come after waves of Isis propaganda that encouraged its supporters to launch strikes on Moscow. Isis propaganda shows bullet holes through Mr Putin's head as well as a poster circulated just days before the attack that showed a falling Kremlin and included the message "We Will Burn Russia".
Some posters on Isis forums linked the explosions to Russia's backing of Syrian president Bashar al-Assad, who is fighting Isis as well as other groups in the Syrian civil war.
The group hasn't yet claimed responsibility for the attack, but often takes as long as a day to do so. If it does claim responsibility for the incident – which it has done with attacks that officials have later said it had no role in – it would be far from the first time it has done so, after it said it had inspired attempted attacks in Chechnya and Russia earlier this year.
The explosion in a St Petersburg Metro train killed at least 10 people and injured 50 more, according to a spokesman for the city's governor. It came as Vladimir Putin was visiting the city, his home town.
As well as saying that investigators were exploring the motive for the attack, Mr Putin offered his condolences to the families of those killed.
The unidentified explosive device went off at 2.20pm local time on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, Russia's National Anti-Terrorism Committee said.
The St Petersburg subway immediately shut down all of its stations and the national anti-terrorism body said security measures would be tightened at all key transport facilities across Russia.
Maxim Liksutov, Moscow's deputy mayor, told Interfax that authorities were tightening security on the subway in the Russian capital.
Social media users posted photographs and video from the Technology Institute subway station in the city centre, showing injured people lying on the floor outside a train with a mangled door.
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