MOSCOW: A suicide bomber killed three policemen and injured another six at a police security checkpoint early on Thursday, Russian investigators said, in the volatile North Caucasus region, where the Kremlin is fighting to staunch an Islamist insurgency.

The suicide bomber detonated explosives in an automobile as police tried to stop the car on its way into the city of Khasavyurt in the province of Dagestan, agencies reported. One person is still unaccounted for.

Rooted in two separatist wars in Chechnya, Islamist insurgents wage daily violence to turn the predominantly Muslim North Caucasus region into an Islamic state. Dagestan has emerged as the epicentre of violence.

"The suicide bomber detonated the explosives when police officers tried to stop the car and check (the person's) documents at a security check point on the way to Khasavyurt," RIA reported a local Interior Ministry statement as saying.

The six injured police had been hospitalised, investigators said.

The Investigative Committee, a criminal investigation arm that answers only to President Vladimir Putin, said the explosion had ripped a four-metre (13-foot) hole in the checkpoint headquarters.

Television showed pictures of police around the security checkpoint, one of many that dots the North Caucasus, where police regularly stop travellers to check identification papers.

Human rights groups say the insurgency, led by self-styled 'Amir' of the North Caucasus Doku Umarov, has been driven by a volatile mix of religion, corruption and grievances against the strong arm tactics of some local leaders against suspected militants and their families.

The suicide bombing follows a police raid on a forest hideout that killed five militants earlier this week in Dagestan.