MOSCOW (REUTERS) - A female suicide bomber attacked a bus in southern Russia on Monday, authorities said, killing at least six people in the deadliest such blast outside the volatile North Caucasus region in nearly three years.

The bombing in Volgograd was likely to raise fears of further attacks by Islamist militants as Russia prepares to host the 2014 Winter Olympics in February in the Black Sea resort city of Sochi, not far from the mainly Muslim North Caucasus.

The attack, which investigators blamed on a 30-year-old woman from Dagestan, the North Caucasus province at the centre of an insurgency, also wounded 32 people, of whom eight were in critical condition, the federal Investigative Committee said.

State television showed footage, taken from a camera mounted on a driver's dashboard, of an explosion ripping through the bus as it travelled along a tree-lined road, sending shards of metal and glass flying.

Passengers scrambled out of doors and windows after the bus had stopped.

"There was a blast - a bang - all the glass flew out of the windows," an eyewitness named Ivan, who had been driving behind the bus, told state-run Rossiya-24 television.

"The cloud of smoke quickly dissipated and then I saw people start to fall out and run out to escape the bus," he said. "It was a horrible sight." Citing a regional investigative source, the Interfax news agency said identity documents belonging to the suspected bomber were found near the site, and that she was believed to have been the wife of an Islamist militant.

The federal Investigative Committee named the suspect as Naida Asiyalova, 30, of Dagestan.

"This woman got on the bus at one of the stops and the explosion occurred almost immediately afterwards. This was confirmed by the surviving passengers," Investigative Committee spokesman Vladimir Markin said.

"The blast was big, it was huge," Vladimir, a man who said his daughter survived the bombing, told Ekho Moskvy radio.

"When I came to pick her up, half the bus was simply not there. It was scary. Very scary," he said.

There was no immediate claim of responsibility.

Volgograd is a city of around 1 million people that lies 900 km southeast of Moscow and a few hundred kilometres north of the North Caucasus and Sochi, at the western end of the Caucasus range, where Russia will hold the Winter Olympics.

President Vladimir Putin has staked his reputation on the Games and ordered authorities to boost security in the North Caucasus, where the Islamist insurgency is rooted in two post-Soviet wars pitting Chechen separatists against the Kremlin.

Insurgents who say they are fighting to create an Islamic state have claimed responsibility for a suicide bombing that killed 37 people at a Moscow airport in 2011 and twin suicide bombings that killed 40 people on the Moscow subway in 2010.

The latter attack was carried out female suicide bombers, dubbed "black widows" in Russia because their male relatives have often been killed by security forces.

In 2002, Chechen women wearing black chadors and suicide belts also took part in a three-day Moscow theatre hostage siege in which around 130 people were killed.