A female suicide bomber has blown herself up on a bus in the southern Russian city of Volgograd, killing at least six people and injuring 30 others, officials say.
The suspected bomber was from Dagestan, one of the predominantly Muslim republics in the North Caucasus, according to Vladimir Markin, the spokesman for the Investigative Committee, Russia's main investigative agency.
In a statement released on Monday, he identified the suspect as 30-year-old Naida Asiyalova based on identity documents found near the explosion site.
The Russian state television showed pictures of Asiyalova's passport.
Rasul Temirbekov, a spokesman for the Investigative Committee's branch in Dagestan, said Asiyalova had a fatal bone illness and was married to an ethnic Russian, Dmitry Sokolov, whom she had met while both were university students in Moscow.
She encouraged Sokolov - also known as Abdul Jabbar - to become a rebel, and he quickly gained a reputation as an expert in explosives, Temirbekov said.
Irina Gogolyeva, a spokeswoman for the Emergency Situations Ministry, said 40 people were on the bus when the explosion occurred on Monday afternoon.
The local government on Monday placed security forces in the Volgograd region on high alert for 15 days.
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 called three months ago for a resumption of attacks on civilians and urged armed fighters to target the Winter Olympics, to be held in Sochi in February 2014.
Volgograd is a city of about one million people that lies 900km southeast of Moscow and a few hundred kilometres north of the North Caucasus and the Black Sea resort city of Sochi.
President Vladimir Putin has staked his reputation on the Games and ordered authorities to boost security in the North Caucasus.
Fighters who say they are fighting to create an Islamic state in Russia's mostly Muslim North Caucasus have carried out deadly bombings inside and outside the region, made up of several provinces along Russia's southern border.
The fighters claimed responsibility for a suicide bombing that killed 37 people at Moscow airport in January 2011, and two nearly simultaneous suicide bombings that killed 40 people on the Moscow subway in 2010.