Tensions were already high after President Donald Trump walked away from the accord a year ago.

But they have been ratcheted up significantly in recent weeks with the US deploying an aircraft carrier strike group and B-52 bombers to the Gulf over alleged threats from Iran.

"The escalation by the United States is unacceptable," Iranian Foreign Minister Mohammad Javad Zarif said in Tokyo Thursday where he is holding talks with Japanese officials.

"We exercise maximum restraint... in spite of the fact that the United States withdrew from JCPOA last May," he added, referring to the agreement on Tehran's nuclear program known as the Joint Comprehensive Plan of Action.

He added that Tehran remains "committed" to the deal, and said continuing assessments showed Iran was in compliance with the multilateral agreement.

Zarif's comments came hours after the US ordered non-emergency staff evacuated from its Baghdad embassy due to an "imminent" threat from Iranian-linked Iraqi militias.

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.

Trump, however, predicted Iran would "soon" want to negotiate and denied there was any discord in the White House over moves that critics say could lead to war in the Middle East.

"I'm sure that Iran will want to talk soon," the president tweeted. He also blasted media reports of White House turmoil, saying "there is no infighting whatsoever. Different opinions are expressed and I make a final and decisive decision."

Opponents of Trump say hardliners led by national security advisor John Bolton, who has long advocated toppling the Iranian regime, are pushing the country into war.

- 'Imminent threat' -

Despite international scepticism, the US government has been pointing to increasing threats from Iran, a long-time enemy and also a 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."

Iran's supreme leader, Ayatollah Ali Khamenei, Tuesday insisted the showdown with 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."

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

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

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

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 defence ministry said Wednesday they have "long been clear about our concerns over Iran's destabilizing behaviour in the region" -- while still not confirming any new imminent danger.

- Tanker 'attack' -

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 incident 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.

Separately, a Saudi-led military coalition Thursday carried out strikes against Yemen's rebel-held capital.

The raids came after Yemen's Iran-aligned Huthi rebels claimed responsibility for drone strikes Tuesday which damaged a Saudi oil pipeline.

Saudi's deputy defence minister, Khalid bin Salman, Thursday accused Iran of ordering the drone attacks.

The Saudi-led coalition intervened in the war in Yemen in March 2015 to push back an advance by the Huthi rebels.