Iran's president says Syrian leader Bashar Assad must remain in power to fight extremist groups like the Islamic State group, and must not be weakened.

Hassan Rouhani said in a meeting with scholars and think tank experts on Sunday evening that Iran will cooperate with any country that puts fighting and defeating "terrorism" as its top priority - but it cannot work with any government that makes a change in Syria's regime its top priority.

Rouhani's view puts Iran at odds with key nations who adopted a road map in Geneva in June 2012 that calls for the formation of a transitional government in Syria.