A dashboard camera has recorded the explosion on a bus in the southern Russian city of Volgograd which authorities suspect was carried out by female suicide bomber.

As least six children were killed, the deadliest such attack outside Russia's volatile North Caucasus region in nearly three years.

The attack, blamed on Russia's mainly Muslim North Caucasus region, came four months before it hosts the Winter Olympics nearby in the Black Sea resort city of Sochi.

The explosion in the city of Volgograd also wounded 32 people, eight of whom were in grave condition, a spokesman for the investigators said.

Citing a source the regional Investigative Committee office, Interfax 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.

There was no immediate claim of responsibility.

Volgograd is a city of around one million people that lies 900 km (560 miles) southeast of Moscow and a few hundred kilometers north of the North Caucasus and the Olympic host venue in Sochi in February.

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 have claimed responsibility for a suicide bombing that killed 37 people at Moscow airport in January, 2011, and twin suicide bombings that killed 40 people on the Moscow subway in 2010.

Dubbed as "Black Widows" in Russia, female suicide bombers spread fear carrying out a string of attacks in the early 2000s. They are thought to be motivated by revenge for the killing of militants husbands by Russian security forces.