Dzhabrail Kostoyev was on his way to work in the main city of Nazran when his car was hit by a blast. Authorities said it may have been a suicide attack.

Mr Kostoyev, who oversaw the Ingush police, had been the target of several previous assassination attempts.

His two bodyguards and four civilians were also killed in Wednesday's attack.

It is thought the civilians were in at least one other passing car that was caught up in the explosion.

"Almost nothing remains from the car that contained the explosive," a police spokesman, quoted by Itar-Tass news agency, said.

'Tough attitude'

Television pictures showed the charred remains of a vehicle on a road littered with debris, and damage to nearby buildings.

The Ingush interior ministry said its initial suspicion fell on criminals.

The Ingush prosecutor, Mahmud-Ali Kalimatov, said it could be a revenge attack.

"He had a tough attitude to bandits. That is why there can be no mistake that he was killed because of the connection with his professional work," he told the BBC.

But the republic has also suffered from the conflict in neighbouring Chechnya and security officials have not yet ruled out that the attack was linked to militant groups in the region.

Instability

Attacks against Russian officials have become commonplace in Ingushetia, Dagestan and other provinces in the North Caucasus near Chechnya.

The worst in Ingushetia occurred in June 2004, when several hundred rebels launched a major attack on police and security officials in Nazran leaving 92 people dead.

Also on Wednesday, the acting chief of a prison was shot dead in another republic of the North Caucasus, Karachayevo-Cherkessiya.

Earlier this week in Dagestan, a dozen Russian security officers were injured and one killed in a shootout with militants. Two rebels also died. The authorities said they had been plotting to seize a school, as happened in Beslan, North Ossetia in 2004.

The BBC's Steve Rosenberg says it is clear that tension across the North Caucasus remains high, and that the Kremlin has been unable to bring peace and stability to its most volatile region.