A female suicide bomber has blown herself up on a bus in the southern Russian city of Volgograd, killing six people and injuring about 30, officials say.
The attack added to security fears ahead of the Winter Olympics in Sochi.
The suspected bomber was from the North Caucasus, a region in southern Russia where an Islamic insurgency has been simmering for more than a decade following two separatist wars in Chechnya. A local official said the suspected attacker was married to an Islamic militant.
No one immediately claimed responsibility for Monday's suicide bombing, but it was the first outside the North Caucasus since Chechen rebel leader Doku Umarov three months ago called for a resumption of attacks on civilians and urged militants to target the Sochi Games, which are to be held in February.
Russia in past years has seen a series of terror attacks on buses, aeroplanes and other forms of transportation, some of them carried out by suicide bombers. The last suicide attack on a bus was in 2008.
The suspected bomber was from Dagestan, one of the predominantly Muslim republics in the North Caucasus, said Vladimir Markin, the spokesman for the Investigative Committee, Russia's main investigative agency.
In a statement, he identified the suspect as 30-year-old Naida Asiyalova. Russian state television showed pictures of Asiyalova's passport.
Rasul Temirbekov, a spokesman for the Investigative Committee's branch in Dagestan, said the suspected bomber was married to an ethnic Russian, Dmitry Sokolov, whom she had met while both were university students in Moscow.
She encouraged him to become a rebel, and he quickly gained a reputation as an expert in explosives, Temirbekov said. Sokolov, whose nom de guerre is Abdul Jabbar, has been on the run.
Temirbekov said that the suspected suicide bomber had a fatal bone illness.
Video broadcast on state Rossiya television showed that Monday's explosion occurred as the bus was moving in the far right lane of a divided six-lane road. The video, taken from a vehicle travelling behind the bus, showed a burst of flame and grey smoke. Fragments of what appeared to be window frames and other parts of the bus were left scattered across the road.
When the bus came to a stop, the video showed many passengers jumping out.
Markin said seven people died, including the suicide bomber, and 33 asked for medical assistance, 28 of whom were hospitalised. Emergency officials said about 40 people were on the bus.