MAKHACHKALA : A Muslim cleric in Russia's restive Dagestan was shot dead in an ambush, investigators said Sunday, the latest in a string of murders of moderate religious figures in a province plagued by an Islamist insurgency.

Gunmen opened fire on 66-year-old Ilyas Ilyasov on Saturday as he was getting into his car near his home in Makhachkala, the largest city in the region, Russia's Investigative Committee said in a statement. Ilyasov died on the spot, while his driver was injured and hospitalised, investigators said.

"The preliminary theory is (the victim's) religious activities," investigator Rasul Temirbekov told a Russian television channel. Ilyasov was well-known in Dagestan as an intellectual, a skilled orator, and a mediator who had resolved several high-profile conflicts in the region.

Russian authorities are struggling to combat Islamist extremists who attack policemen almost daily, with killings of clerics and journalists also occurring with disturbing regularity.

Ilyasov was an imam at a popular mosque in western Makhachkala who frequently gave sermons across Dagestan.

Despite often expressing criticism of the government's policies at rallies, Ilyasov was respected by officials, including the former regional chief, Magomedsalam Magomedov.

The mountainous Muslim-majority region on the Caspian sea has become the most violent in the North Caucasus.

Last year a suicide bomber killed one of Dagestan's most revered clerics, Said Afandi, in a shocking attack inside his home that left seven dead.