Militiamen and their supporters protesting against deadly US air strikes on Iraq have hurled stones and torched a security post at the US Embassy in Baghdad in an unprecedented attack on an American diplomatic mission in the country.

Embassy guards responded with stun grenades and tear gas after the attackers stormed and burned the security post at the entrance on Tuesday but did not breach the main compound.

The incident marked a sharp escalation of the conflict between Washington and Tehran – both influential players in Iraq – while mass protests are challenging Iraq’s own political system nearly 17 years after the US invasion that toppled Saddam Hussein.

It comes after months of security incidents in the region for which both sides have traded blame.

US President Donald Trump accused Iran of orchestrating the violence and said it would be held responsible. Iran rejected the accusation.

More US troops were being sent to the embassy, US officials said.

The US Embassy in Baghdad has been hit by sporadic but non-lethal rocket fire in recent months, and was regularly shelled following the US-led invasion of 2003, but has not been physically attacked by demonstrators in this way before.

The protests were led by Iranian-backed militias and lasted several hours, but the US State Department said later that personnel were secure and the facility had not been breached.

A State Department spokesperson said reports from Iraqi officials that the ambassador had been evacuated were false. The envoy, Matt Tueller, had been on previously scheduled personal travel and was returning to the embassy, the official said. There were no plans for an evacuation.

US air strikes on Sunday on Iranian-backed militia bases killed at least 25 fighters and wounded 55.

Those raids, on the Kataib Hezbollah militia, were in response to the killing of a US civilian contractor in a rocket attack on an Iraqi military base.

“Iran killed an American contractor, wounding many. We strongly responded, and always will,” Donald Trump said in a tweet. “Now Iran is orchestrating an attack on the US Embassy in Iraq. They will be held fully responsible.”

….Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities. They will pay a very BIG PRICE! This is not a Warning, it is a Threat. Happy New Year!

— Donald J. Trump (@realDonaldTrump) December 31, 2019

“America has the surprising audacity of attributing to Iran the protests of the Iraqi people against (Washington’s) savage killing of at least 25 Iraqis,” Iranian foreign ministry spokesman Abbas Mousavi said.

The protesters, who were joined briefly by Iranian-backed Shi’ite Muslim militia leaders, threw stones at the gate while others chanted, “No, no, America! No, no, Trump!”

Iraqi special forces were deployed around the main gate to prevent protesters entering. US-trained Iraqi Counter Terrorism forces later reinforced them.

The Popular Mobilisation Forces, an umbrella grouping of the militias which have been officially integrated into Iraq’s armed forces, said 62 militiamen and civilians were wounded by the tear gas and stun grenades fired to disperse the crowd.

Donald Trump spoke with Iraqi Prime Minister Adel Abdul Mahdi and emphasised the need to protect US personnel and facilities, the White House said.

Iraqi President Barham Salih denounced the attempts to breach the embassy and said security forces would take their duties to protect foreign missions seriously.

-AAP