President Donald Trump signed an executive order barring citizens of Iraq, Syria, Sudan, Iran, Somalia, Libya and Yemen from entering the US for at least 90 days, a move he billed as an effort to make America safe from "radical Islamic terrorists".
Both units from the Hashed al-Shaabi paramilitary organisation and American troops are deployed in the Mosul area as part of the operation to retake the city from the Islamic State group, and heightened anti-US sentiment among militiamen could increase the danger to Washington's forces.
The statement did not specify if the call applied to American military personnel in Iraq, and a spokesman was unreachable for comment.
The most powerful groups in the Hashed al-Shaabi are Iran-backed Shiite militias, some of whom fought against US forces in previous years.
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Thousands of American troops are deployed to Iraq as part of a US-led coalition against IS that has provided air support, training and other assistance to Baghdad's forces.
But they have also faced repeated accusations of abuses including summary executions, kidnappings and destruction of property in the course of the war against IS.
Trump's decision led to the detention of incoming refugees at US airports, sparking protests, legal challenges and widespread condemnation from rights groups.
The travel restrictions, which come on the heels of repeated assertions by Trump that the US should have stolen Iraq's oil before leaving in 2011, risk alienating the citizens and government of a country fighting against militants the president has cast as a major threat to America.