A Shiite militant group is threatening US interests in Iraq as part of the backlash over an anti-Islam film it has described as "heinous."

The leader of the Iranian-backed Asaib Ahl al-Haq said in an interview on Thursday that the amateurish video that ridicules Prophet Muhammad was unforgiveable, and called on all Muslims to "face our joint enemy."

The militia leader, Qais al-Khazali, told The Associated Press that the video "will put all the American interests Iraq in danger."

The US Embassy in Baghdad remains the world's largest American diplomatic mission, with an estimated 15,000 employees.

Asaib Ahl al-Haq led deadly attacks against US troops before the American military withdrew from the country in 2011. It is funded and armed mainly by Iran.