Iranian Foreign Minister Mohammad Javad Zarif is "one of the "front men for a corrupt religious mafia," top US diplomat Mike Pompeo said Monday following his surprise resignation.

"Either way, he and @HassanRouhani are just front men for a corrupt religious mafia," Pompeo wrote, referring to the country's president.

"We know @khamenei_ir makes all final decisions. Our policy is unchanged -- the regime must behave like a normal country and respect its people," he said of Iran's supreme leader Ayatollah Ali Khamenei.

Zarif, 59, announced his resignation on Instagram, but it can only take effect once Rouhani accepts it.

He has served as Rouhani's foreign minister since August 2013 and has been under constant pressure and criticism by hardliners who opposed his policy of detente with the West.

His standing within Iran's political establishment took a hit when the US withdrew last year from a deal aimed at curbing Tehran's nuclear program, and the deal's achievements became less and less clear as Iran's economy nosedived.

Zarif was blamed by ultra-conservatives for negotiating a bad deal that had not gained anything meaningful for Iran in exchange for all the concessions it had made in its nuclear program.

The faceoff between the minister and his critics only intensified as time passed, with Zarif saying his main worry throughout the nuclear talks had been from pressure inside Iran.

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