Washington ordered non-emergency government workers out of Iraq on Wednesday amid rising tensions between the U.S. and Iran.
A State Department spokesperson said the withdrawal was based on a security assessment.
And it comes after Reuters learned America grew alarmed about intelligence reports suggesting Iranian-backed forces in Iraq shifted rockets near bases housing U.S. forces.
Two Iraqi security sources told Reuters that Secretary of State Mike Pompeo used a secret trip to Baghdad last month to tell Iraqi officials to keep the Shiite militias in check.
If not, the U.S. would respond with force.
A senior Iraqi military source told Reuters, "The message from the Americans was clear. They wanted guarantees that Iraq would stop those groups threatening U.S. interests."
Adding, "they said if the U.S. were attacked on Iraqi soil, it would take action to defend itself without coordinating with Baghdad."
Iraqi Prime Minister Adel Abdul Mahdi on Tuesday (May 14) told reporters Baghdad had not observed a threat to Americans and hoped tensions would ease.
(SOUNDBITE)(Arabic) IRAQI PRIME MINISTER ADEL ABDUL MAHDI, SAYING (TUESDAY):
''We hope that things will end well and there are indications from both parties that things will end well and there is a real desire to find outlets for this crisis.''
Armed Shiite paramilitary groups, many backed by Iran, played a key role in retaking territory from Islamic State.
And many of these forces are now part of the Iraqi security apparatus. But some are more or less independent of Baghdad's control, and play in role in maintaining Iranian influence in Iraq.
Washington warned last month of non-specific threats from Iranian-backed groups.
The Pentagon deployed the U.S.S. Abraham Lincoln carrier group to the region, along with B-52 bombers.
President Donald Trump on Tuesday denied a report that the U.S. was considering sending 120,000 soldiers to the Middle East.
(SOUNDBITE) (English) U.S. PRESIDENT DONALD TRUMP SAYING (TUESDAY):
"Would I do that? Absolutely. But we have not planned for that. Hopefully we're not going to have to plan for that. And if we did that, we'd send a hell of a lot more troops than that."
Iran's Supreme Leader on Tuesday said Tehran did not want conflict with the U.S.
(SOUNDBITE) (Farsi) IRAN SUPREME LEADER, AYATOLLAH ALI KHAMENEI, SAYING (OVERLAID WITH SHOTS OF AUDIENCE LISTENING):
"There won't be any war, with the help of God. We don't seek a war, and they don't either. They know it's not in their interests."
But Ayatollah Ali Khamenei also said there was no point in holding negotiations with this White House.