A suicide bomber has set off a blast in a train station in the southern Russian city of Volgograd killing at least 15 people and injuring at least 34, news reports and officials have said.

Regional Interior Ministry spokeswoman Svetlana Smolyaninova told the ITAR-TASS news agency on Sunday that the blast occurred inside the city's main train station at around 12:45pm (08;45 GMT).

Vladimir Markin, the spokesman for the nation's top investigative agency, the Investigative Committee, said the suicide bomber detonated her, the bomber was initially believed to be a women but officials later said this was not certain, explosives in front of a metal detector.

"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," Markin said in a statement.

He added that the bomb contained about 10 kilograms of TNT explosives, and was rigged with shrapnel.

Markin said that security controls prevented a far greater number of casualties at the station, which was packed with people at a time when several trains were delayed.

Sergei Bozhenov, Volgograd's regional governor, said 15 died. Russia's Health Ministry said at least 34 people were injured.

Al Jazeera's Peter Sharp, reporting from Moscow, said one of those injured was a nine-year old girl, who was shielded by her mother, who died in the explosion.

Throughout the day, officials issued conflicting statements on casualties. They also said that the suspected bomber was a woman, but then reversed said the attacker could have been a man.

'Black widows'

The city of Volgograd, known as Stalingrad in the Soviet era, was attacked in October by a female suicide bomber with links to groups fighting federal forces in Russia's volatile North Caucasus.

The October 21 strike killed six people aboard a crowded bus and immediately raised security fears ahead of the February 4-23 Winter Games in Sochi.

The Black Sea city that will host the Olympics lies 690km southwest of Volgograd and in direct proximity of the violence ravaging North Caucasus regions such as Dagestan and Chechnya on a daily basis.

"It is almost certain that Vladimir Putin will be meeting people in the Kremlin to revise and go over the security operation that's already in place," Al Jazeera's Sharp said.

More than 30,000 troops and security personnel have been drafted in the Sochi region ahead of the games, our correspondent said.

Fighters are said to be seeking to impose an Islamist state throughout Russia's North Caucasus.

Their leader Doku Umarov has ordered his foot soldiers to target civilians outside the region and disrupt the Olympic Games.

Female suicide bombers are often referred to in Russia as "black widows", women who seek to avenge the deaths of their family members in North Caucasus fighting by targeting Russian civilians.

Female suicide bombers set off blasts at two Moscow metro stations in March 2010 that killed more than 35 people.

They were also responsible for taking down two passenger jets that took off from a Moscow airport within minutes of each other in 2004, killing about 90 people.