Police in the southern Russian province of Dagestan said Friday that a Muslim cleric, who criticized radical Islamists, has been killed by a powerful explosion.

Gitinmagomed Abdulgapurov, an imam in a mosque in the central city of Buinaksk, became the fifth Muslim cleric in the restive province killed in the past two years.

Islamic radicals and insurgents launch frequent attacks on security forces and officials.

Dagestan Interior Ministry spokesman Vyacheslav Gasanov said Abdulgapurov was killed early Friday along with his bodyguard when they walked to the mosque for a prayer.

He said the makeshift explosive device, hidden in a heap of garbage and operated by a cell phone, killed the men instantly.

Moscow-based investigators said in a statement that the imam had been placed under police protection after receiving death threats earlier.

Islamist insurgency has spread across Dagestan, a Muslim Caspian Sea region known for its ancient culture and tapestry of ethnic groups, and other provinces in Russia's Caucasus after two separatist wars in Chechnya.