A female suicide bomber has been blamed for killing 15 people and leaving dozens more injured at a railway station in the southern Russian city of Volgograd.
Police identified the bomber as a Dagestan national called Oksana Aslanova – who had been married to two Islamists killed by Russian forces.
She apparently detonated a bomb in front of a metal detector inside the main entrance of the station. Russian television is suggesting there may have been two attackers.
Russian President Vladimir Putin ordered law enforcement agencies to take “all necessary measures”.
Federal police spokesman Vladimir Kolesnikov said security would be stepped up at train stations and airports.
One witness, who gave his name as Vladimir, told a Russian TV station: “I heard the blast and ran toward it. I saw melted, twisted bits of metal, broken glass and bodies lying on the street.”
Train station store attendant Valentina Petrichenko said: “It was a very powerful blast. Some people started running and others were thrown back by the wave of the blast. It was very scary.”
The attack comes just weeks before the start of the Winter Olympics in Sochi, a town 430 miles southwest of Volgograd.
Sky News Moscow producer Yulia Bragina said: “It’s very concerning with two months to go to the Sochi Olympics.”
“The Russian security services are working 24/7 to make the Games safe for the sportsmen and people who want to visit the Olympics in Russia. There will be a big investigation into what has happened.”
Aslanova reportedly trained alongside Islamic bomber Naida Asiyalova, 30, who killed seven people in Volgograd in October, one of the deadliest such attacks outside the troubled North Caucasus region in more than two years.
Known as Stalingrad in Soviet times and previously as Tsaritsyn, Volgograd is an important industrial centre with a population of over a million.
On Friday, a car bomb killed three people in the southern city of Pyatigorsk, 170 miles east of Sochi.
In July, Doku Umarov, one of the leaders of an ongoing insurgency in the North Caucasus, an area close to Sochi, urged militants to use “maximum force” to disrupt the Winter Olympics, a project close to Mr Putin’s heart.