Nasr al-Shomari, a military commander for the Iran-backed Harakat al-Nujaba, told AFP the claim was "a pretext" by Washington to create "an uproar" in Iraq.

A leader of the pro-Iran Asaib Ahl al-Haq group, Layth al-Azari, said the allegations were part of a "psychological war" by the United States.

The two groups are key factions within the Shiite-dominated Hashed al-Shaabi organisation which played a key role in the battle to defeat the Islamic State jihadist group in Iraq.

On Wednesday the United States ordered the evacuation of non-emergency staff from its Baghdad embassy, citing an "imminent" threat from Iranian-linked armed groups in Iraq.

It came as tensions soared in the region amid a stand-off between Washington and Tehran.

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 alleged plot by Tehran to attack US forces or allies.

Senior US 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.

But Shomari dismissed the allegations, telling AFP the United States "is trying to create an uproar in Iraq and in the region under any pretext".

"If we put out a statement concerning the United States, they consider it a threat, but if the United States carries out an attack, isn't that a threat?," he added.

Azari echoed his remarks, saying the US claim and its recent action in the region "are a provocation aimed at escalating a psychological war" due to the prevailing tensions with Iran.

Shiite-majority Iraq has been under pressure from the US to limit its ties with neighbouring Iran, particularly after Washington last year withdrew from the 2015 nuclear deal with Tehran and hit it with sanctions.

Iranian Foreign Minister Mohammad Javad Zarif on Thursday accused the United States of an "unacceptable" escalation of tensions.

Zarif, who is in Japan and is set to visit China on Friday, said Iran was exercising "maximum restraint".