This follows last week's rocket attacks on Taji base just north of Baghdad, which houses US troops. At least two Americans have been killed in the recent attacks, blamed on Iran-backed militias, especially Kataib Hezbollah.

Pompeo told Iraqi PM Adil Abd al-Mahdi in a phone call that Baghdad "must defend Coalition personnel supporting the Iraqi government's efforts to defeat ISIS," according to a Monday State Dept. press release.

Those "responsible for the attacks must be held accountable," the statement warned. The US "will not tolerate attacks and threats to American lives" and will take "military action as necessary in self-defense," it added.