Iraqi militia supporters tear-gassed after storming US Baghdad embassy

United States troops fired tear gas on Wednesday as Iran-backed militiamen and supporters gathered outside the American Embassy in Baghdad for a second day.

The protesters had already set fire to the roof of a reception area inside the embassy compound, sparking an angry response from Washington over the lapse in security.

Dozens of Iran-allied militiamen and their supporters had camped out at the gates of the embassy overnight, a day after they broke into the compound, trashing a reception area and smashing windows before pulling back.

It was one of the worst attacks on a US diplomatic mission in years.

US Marines guarding the embassy fired tear gas Wednesday as more crowds arrived and after the protesters lit a fire on the roof of the reception area. There have been no reports of any injuries since the protests began.

Iraqi militiamen were protesting deadly weekend air strikes killing 25 Iran-backed fighters. Those strikes were in response to a rocket attack on an Iraqi army base that killed a US contractor.

US President Donald Trump blamed Iran for the attack on the embassy and Defence Secretary Mark Esper later announced the immediate deployment of about 750 soldiers from the Army's 82nd Airborne Division to the Middle East.

He did not specify their destination, but a US official familiar with the decision said they will go to Kuwait.

Iran has denied any involvement in the attack on the embassy. Foreign Ministry spokesman Abbas Mousavi was quoted by state media on Tuesday as warning the US against any "miscalculation" in the worsening standoff.

Iran's Supreme Leader, Ayatollah Ali Khamenei, criticised the US airstrikes on the Iran-backed Iraqi militia on Sunday.

In remarks carried by the semi-official ISNA news agency, he accused the US of taking revenge on Iran for the defeat of the Islamic State group, which accused of being an "American creation". 

On Wednesday, he issued a direct threat to the US via Twitter.

"If the Islamic Republic decides to challenge & fight, it will do so unequivocally. We’re not after wars, but we strongly defend the Iranian nation’s interests, dignity, & glory," he tweeted.

"If anyone threatens that, we will unhesitatingly confront & strike them."

Read more: Trump sets 'red line' for Tehran amid mounting risks

The political influence of the Iran-backed militias, known as the Popular Mobilisation Forces (PMF) or Hashd Al-Shaabi, has risen in recent years, and their allies dominate the parliament and the government.

That has made them the target of mass protests since October that are unrelated to the attack on the embassy.

The anti-government protesters have attacked Iranian diplomatic missions and the local headquarters of parties affiliated with the militias across southern Iraq.

They have also set up a major protest camp in central Baghdad.

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For weeks, the anti-government protesters have been trying to enter the Green Zone housing the government and the US Embassy, but have been beaten back by security forces, who have killed hundreds of demonstrators.

The militiamen and their supporters, however, were able to quickly enter the Green Zone and mass in front of the embassy, with little if any resistance from authorities, drawing criticism from anti-government supporters. 

Agencies contributed to this report.

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