The move added to growing fears that the long-time rivals could be on course for conflict despite both sides stressing they have no desire for war.

The evacuation order, also covering the US consulate in Arbil, came 10 days after the Pentagon deployed an aircraft carrier task force and B-52 bombers to the Gulf to fend off an unspecified plot by Tehran to attack US forces or allies.

Trump sought to portray the situation as under control, saying there was no discord in the White House and that Iran would want to negotiate.

"I'm sure that Iran will want to talk soon," he tweeted. He also blasted media reports of White House turmoil, saying "there is no infighting whatsoever."

Despite international skepticism, the US government has been pointing to increasing threats from Iran, a long-time enemy and rival of US allies Israel and Saudi Arabia.

Senior State Department officials, speaking on condition of anonymity, said the threat came from Iraqi militia "commanded and controlled" by Tehran's Islamic Revolutionary Guard Corps.

"It is directly linked to Iran, multiple threat streams directly linked to Iran," said one official.

"This is an imminent threat to our personnel," said a second official.

"There is no doubt in my mind that under the circumstances, a partial ordered departure (from the embassy) is a reasonable thing to do."

- 'Maximum restraint' -

On Tuesday, Iran's supreme leader, Ayatollah Ali Khamenei, insisted the showdown between the Islamic republic and the United States was a mere test of resolve.

"This face-off is not military because there is not going to be any war. Neither we nor them (the US) seek war," he said.

US Secretary of State Mike Pompeo echoed that sentiment, saying in Sochi, Russia: "We fundamentally do not seek a war with Iran."

Iran's Foreign Minister Mohammad Javad Zarif said on a visit to Tokyo that his country was exercising "maximum restraint", accusing Washington of an "unacceptable" escalation in tensions.

Tehran remains "committed" to the nuclear deal thrashed out with world powers despite Washington's withdrawal, Zarif said, adding that Iran had been assessed as in compliance with the multilateral agreement.

Despite the insistence that neither party wants conflict, world powers have rushed to urge calm and voiced concern over the escalating tensions.

- Allies skeptical -

Washington says it has received intelligence on possible attacks by Iranian or Iranian-backed forces, possibly targeting US bases in Iraq or Syria.

Some observers speculate Tehran is seeking to retaliate over Washington's decision in April to put Iran's Revolutionary Guards on a terror blacklist -- a move designed to stymie their activities across the Middle East.

But since the first US warning on May 5, the only activity has been a still-mysterious "attack" Monday on tankers anchored off Fujairah, an Emirati port located at the strategically crucial entrance to the Gulf.

One or more vessels incurred light hull damage, but what caused the damage and who was behind it remains unknown.

Britain's Major General Chris Ghika, a spokesman for Operation Inherent Resolve, the coalition fighting the jihadist Islamic State group, said Tuesday there was no special heightened alert.

After Ghika's comments drew a sharp retort from the US Central Command, Britain's defense ministry said Wednesday they have "long been clear about our concerns over Iran's destabilizing behavior in the region" -- while still not confirming any new imminent danger.

Germany and the Netherlands said Wednesday they were suspending training of soldiers in Iraq. German defense ministry spokesman Jens Flosdorff said there was a "generally heightened alert" in the region but gave no specifics.

In the US Congress, Democrats demanded to know why the Trump administration was boosting its Gulf presence and, according to media reports, considering war plans that would involve sending 120,000 US troops to the Middle East if Iran attacks American assets.

Senator Bob Menendez, the senior Democrat on the Senate foreign relations panel, demanded a briefing "immediately" on the threat intelligence, any plans for war and the decision to order embassy staff out of Iraq.

"Congress has not authorized war with Iran... If (the administration) were contemplating military action with Iran, it must come to Congress to seek approval," he said.