A female suicide bomber killed at least 13 people at a train station in the Russian city of Volgograd on Sunday, authorities said, in the deadliest such blast outside the volatile North Caucasus region in nearly three years.
The second deadly attack in southern Russia in three days will bolster fears of attacks by Islamist militants as Russia prepares to host the 2014 Winter Olympics in the Black Sea city of Sochi in less than six weeks' time.
The suicide bomber detonated her explosives in front of a metal detector just inside the main entrance of the train station, the federal Investigative Committee said. It said at least 13 people were killed and the number of wounded was not yet known.
As many as 50 people were wounded in the blast that blew out the windows of the lower floors of the imposing columned building, a health ministry spokesman told Rossiya-24 television.
TV footage showed emergency services carrying out victims, with at least one body lying motionless on the ground.
"I heard the blast and ran toward it," a witness, Vladimir, told Rossiya-24. "I saw melted, twisted bits of metal, broken glass and bodies lying on the street."
Volgograd is a city of around 1 million people, about 430 miles (690 km) northeast of Sochi, where the Winter Olympics - a major prestige project for President Vladimir Putin - will open on Feb 7.
It lies close to Russia's North Caucasus, a strip of mostly Muslim provinces plagued by near-daily violence in a long-running Islamist insurgency. Insurgent leader Doku Umarov, a Chechen warlord, urged militants in a video posted online in July to use "maximum force" to prevent Putin staging the Olympics.
An attack by a female suicide bomber killed seven people in Volgograd on Oct. 21.
On Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk, 270 km (170 miles) east of Sochi.