Shiite-majority Iran has been helping Iraqi, Syrian and Kurdish forces battle the Sunni extremist group on the ground while the U.S.-led coalition has been bombing it from the air. The Islamic State group views Shiites as apostates deserving of death and has massacred hundreds of captured Syrian and Iraqi soldiers, as well as Sunni rivals.

Grand Ayatollah Nasser Makarem Shirazi, the chief organizer of the conference, appealed for consensus among Islam's two main branches, urging all Muslim clerics to work to discredit groups espousing extremism.

"Military attacks against this deviant group (IS) are necessary but insufficient. The roots of their violent ideology must be dried up. This is the job of Muslim scholars, to preach the true, moderate face of Islam and expose the ugly face of IS ideology," said Shirazi, a prominent Shiite cleric who has a large following in Iran and abroad.

Iraq's Foreign Minister Ibrahim al-Jaafari, a Shiite, said the Islamic State group is the biggest threat to Islam. "They were created to undermine Islam and destroy Muslim societies. IS kills both Shiite and Sunni Muslims," he said.

Sunni scholar Abdolrahman Sarbazi, who leads Friday prayers in an area of southeastern Iran that is home to many Sunnis, said "Sunni Muslims also condemn the violent practices of takfiris, who are a threat to humanity." "Takfiri" is an Arabic word for Islamic extremist.