The evacuation order came 10 days after President Donald Trump ordered the movement of 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.

Both Washington and Tehran have said they have no intention of going to war, but tensions have soared in recent weeks.

Senior State Department officials, speaking on condition of anonymity, said that the threat came from Iraqi militias "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 is a reasonable thing to do."

- War plans denied -

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

"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."

But the evacuation of US diplomatic personnel from Iraq added to broader worries about a possible armed clash ahead.

Democrats in Congress demanded the Trump administration brief them on the Iran threat, warning that the US legislature has not approved military action against Tehran.

On Monday, British foreign minister Jeremy Hunt called for "a period of calm" and bluntly warned of the danger of pushing Iran back towards developing nuclear weapons.

And in Moscow, President Vladimir Putin's spokesman Dmitry Peskov on Wednesday expressed concerns about "the continued escalation of tensions around this subject."

- 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 that 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 seen 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.

US allies continued to show skepticism over Washington's alarm bells.

Major General Chris Ghika, a British spokesman for Operation Inherent Resolve, the coalition fighting the Islamic State group, said Tuesday there was no special heightened alert, adding that OIR troops were always on guard against possible attacks.

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 "generally heightened alert, awareness" among soldiers 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.