Iran and the United States have a shared opposition to IS, which controls a swathe of both Iraq and neighbouring Syria, but both governments deny cooperating militarily against the jihadists.

“There is still no serious understanding about the threat and they (the United States) have as yet taken no serious action,” Zarif was quoted as saying by Iran’s Mehr news agency.

“They have helped (IS) in Syria in different ways,” he added, alluding to support that the United States has provided to some rebel groups fighting President Bashar al-Assad’s forces.