The United States is threatening to close its embassy in Baghdad amid frustrations over attacks by Shi'ite militias.

News organizations quoted U.S. and Iraqi officials Sunday as saying U.S. Secretary of State Mike Pompeo warned Iraqi leaders in recent days that the embassy would close unless Iraq worked to stop attacks on U.S. military and diplomatic sites.

Pompeo has not discussed the matter publicly, and a State Department spokesperson declined Monday to comment on the secretary's conversations with foreign leaders.

But the spokesperson told reporters the United States "will not tolerate threats to our men and women serving abroad."

"We have made the point before that the actions of lawless Iran-backed militias remain the single biggest deterrent to stability in Iraq," the spokesperson said. "It is unacceptable for Iran-backed groups to launch rockets at our embassy, attack American and other diplomats, and threaten law and order in Iraq."

The United States has demanded Iraqi action before, saying in January that Iraq has an obligation to protect U.S. diplomatic facilities in the country.

That message came weeks after the U.S. military responded to a series of attacks against U.S. personnel in Iraq with an airstrike that killed Iran's top general, Qassem Soleimani, and senior Iraqi militia commander Abu Mahdi al-Muhandis, outside the Baghdad airport.

The strike sparked a round of protests in Iraq, with demonstrators, and several Iraqi lawmakers, objecting to the U.S. action as a violation of Iraqi sovereignty and calling on the government to expel U.S. troops.

The U.S. forces are in Iraq to advise and assist Iraqi forces in their missions against Islamic State militants. The Pentagon announced earlier this month the U.S. troop presence would drop from 5,200 to 3,000.