BAGHDAD - Security forces at the US embassy in Baghdad fired tear gas early Wednesday to disperse pro-Iran demonstrators who had spent the night outside the mission's gates, an AFP photographer said.

Thousands of supporters of Iraqi armed factions close to Iran had massed at the embassy on Tuesday in outrage over US air strikes that killed 25 pro-Iran fighters at the weekend.

They marched easily through checkpoints in the usually high-security Green Zone to reach the embassy, broke through a security reception area and scribbled graffiti in support of Tehran.

Setting up some 50 tents and even portable bathrooms, hundreds of demonstrators announced a sit-in at the embassy's gates until US forces are ousted from the country.

On Wednesday morning, the crowds drew closer to the single wall separating them and the larger diplomatic compound, setting US flags on fire.

In response, security forces inside fired tear gas at the protesters, several of whom were reported wounded and ferried away by ambulances.

A cordon of Iraqi security forces that had formed around the embassy dissipated when the gas was fired.

But reinforcements from the federal police soon arrived and the brief skirmishes calmed.

The US has blamed Iran for the attack but also pressured Iraqi security forces to step up, alarmed that the mobs were able to reach the embassy so easily.

Defense Secretary Mark Esper said around 750 troops from a rapid response unit of the 82nd Airborne Division were prepared to deploy over the next several days to the region in response to the unrest.

Prior to the announcement, a US official told AFP that "up to 4,000 (troops) may ultimately be deployed".

Overnight, the US flew a rapid response team of Marines into the embassy to reinforce security there.

No US personnel were injured and US officials said there were no plans to evacuate the mission. -AFP