An imam has been killed in a mosque in Russia's southern Dagestan region, police said on Friday.

The four masked assailants also killed another man and set the mosque on fire, before fleeing the scene in a stolen car.

The imam, Magomedkamil Gamzatov, was well-known for his rejection of "any form of violence and openly condemned the extremist insurgency," the National Anti-Terror Committee said in a statement.

Dagestan, next to volatile Chechnya in the North Caucasus, sees frequent attacks blamed by Moscow on Islamic militants. Security forces are searching for the assailants.