Iraqi supporters of an Iranian-backed militia have stormed the US embassy compound in Baghdad and set it on fire during a demonstration that has raised tensions between Tehran and Washington, and forced the American administration to send in military reinforcements.

The protesters broke down the US embassy gate door and managed to make their way into the security kiosk at the entrance of the heavily guarded compound, as gunshots and sirens rang out in the Iraqi capital, according to news organisations and video footage.

Security forces fired tear gas and stun guns in an attempt to disperse the hundreds of protesters, injuring at least two militiamen. The guard towers protecting the massive compound also appeared to be on fire.

US president Donald Trump blamed Iran for both the attack on the military facility and the embassy attack.

“Iran killed an American contractor, wounding many. We strongly responded, and always will,” he wrote on Twitter. “Now Iran is orchestrating an attack on the US embassy in Iraq. They will be held fully responsible. In addition, we expect Iraq to use its forces to protect the embassy, and so notified!”

Baghdad embassy attack: in pictures Show all 21 1 /21 Baghdad embassy attack: in pictures Baghdad embassy attack: in pictures Protesters and militia fighters gather to condemn air strikes on bases belonging to Hashd al-Shaabi (paramilitary forces), outside the main gate of the U.S. Embassy in Baghdad, Iraq December 31, 2019 REUTERS Baghdad embassy attack: in pictures Hashd al-Shaabi (paramilitary forces) fighters set fire on the U.S. Embassy wall to condemn air strikes on their bases, in Baghdad, Iraq December 31, 2019 REUTERS Baghdad embassy attack: in pictures Hashd al-Shaabi (paramilitary forces) fighters set fire on the U.S. Embassy wall to condemn air strikes on their bases, in Baghdad, Iraq December 31, 2019 REUTERS Baghdad embassy attack: in pictures Hashd al-Shaabi (paramilitary forces) fighters set fire on the U.S. Embassy wall to condemn air strikes on their bases, in Baghdad, Iraq December 31, 2019 REUTERS Baghdad embassy attack: in pictures Iraqi protesters set ablaze a sentry box in front of the US embassy building in the capital Baghdad to protest against the weekend's air strikes by US planes on several bases belonging to the Hezbollah brigades near Al-Qaim, an Iraqi district bordering Syria, on December 31, 2019. - Several thousand Iraqi protesters attacked the US embassy in Baghdad on today, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. AFP via Getty Images Baghdad embassy attack: in pictures Several thousand Iraqi protesters, waving flags in support of Iraq's Hezbollah movement and the Hashed al-Shaabi, a mostly Shiite network of local armed groups trained and armed by powerful neighbour Iran, demonstrate outside the US embassy in Baghdad on December 31, 2019, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. - It was the first time in years protesters have been able to reach the US embassy in the Iraqi capital, which is sheltered behind a series of checkpoints in the high-security Green Zone. AFP via Getty Images Baghdad embassy attack: in pictures Protesters and Iraqi Shiite militia supporters gather outside the US Embassy during a protest in Baghdad, Iraq, 31 December 2019. According to media reports, the US ambassador and other members of the staff were evacuated as dozens of people broke into the embassy's compound after setting fire to the reception area. The attack at the embassy follows a deadly US airstrike that killed 25 fighters of the Iran-backed militia on 29 December. EPA Baghdad embassy attack: in pictures Protesters and Iraqi Shiite militia supporters gather outside the US Embassy during a protest in Baghdad, Iraq, 31 December 2019. According to media reports, the US ambassador and other members of the staff were evacuated as dozens of people broke into the embassy's compound after setting fire to the reception area. The attack at the embassy follows a deadly US airstrike that killed 25 fighters of the Iran-backed militia on 29 December. EPA Baghdad embassy attack: in pictures Several thousand Iraqi protesters, waving flags in support of Iraq's Hezbollah movement (L) and the Hashed al-Shaabi (C), a mostly Shiite network of local armed groups trained and armed by powerful neighbour Iran, demonstrate outside the US embassy in Baghdad on December 31, 2019, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. - It was the first time in years protesters have been able to reach the US embassy in the Iraqi capital, which is sheltered behind a series of checkpoints in the high-security Green Zone. AFP via Getty Images Baghdad embassy attack: in pictures A Hashd al-Shaabi fighter holds a sign reading "Close the American embassy otherwise people will close it", as others set fire on the U.S. Embassy wall to condemn air strikes on bases belonging to Hashd al-Shaabi (paramilitary forces), in Baghdad, Iraq December 31, 2019. REUTERS Baghdad embassy attack: in pictures Protesters and Iraqi Shiite militia supporters wave flags as they gather outside the US Embassy during a protest in Baghdad, Iraq, 31 December 2019. According to media reports, the US ambassador and other members of the staff were evacuated as dozens of people broke into the embassy's compound after setting fire to the reception area. The attack at the embassy follows a deadly US airstrike that killed 25 fighters of the Iran-backed militia on 29 December. EPA Baghdad embassy attack: in pictures Protesters and Iraqi Shiite militia supporters wave flags as they gather outside the US Embassy during a protest in Baghdad, Iraq, 31 December 2019. According to media reports, the US ambassador and other members of the staff were evacuated as dozens of people broke into the embassy's compound after setting fire to the reception area. The attack at the embassy follows a deadly US airstrike that killed 25 fighters of the Iran-backed militia on 29 December. EPA Baghdad embassy attack: in pictures Protesters and Iraqi Shiite militia supporters wave flags as they gather outside the US Embassy during a protest in Baghdad, Iraq, 31 December 2019. According to media reports, the US ambassador and other members of the staff were evacuated as dozens of people broke into the embassy's compound after setting fire to the reception area. The attack at the embassy follows a deadly US airstrike that killed 25 fighters of the Iran-backed militia on 29 December. EPA Baghdad embassy attack: in pictures Iraqi protesters demonstrate outside the US embassy in Baghdad on December 31, 2019, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. - It was the first time in years protesters have been able to reach the US embassy in the Iraqi capital, which is sheltered behind a series of checkpoints in the high-security Green Zone. AFP via Getty Images Baghdad embassy attack: in pictures Members of the Iraqi pro-Iranian Hashed al-Shaabi group and protesters set ablaze a sentry box in front of the US embassy building in the capital Baghdad to protest against the weekend's air strikes by US planes on several bases belonging to the Hezbollah brigades near Al-Qaim, an Iraqi district bordering Syria, on December 31, 2019. - Several thousand Iraqi protesters attacked the US embassy in Baghdad on today, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. AFP via Getty Images Baghdad embassy attack: in pictures Members of the Iraqi pro-Iranian Hashed al-Shaabi group and protesters set ablaze a sentry box in front of the US embassy building in the capital Baghdad to protest against the weekend's air strikes by US planes on several bases belonging to the Hezbollah brigades near Al-Qaim, an Iraqi district bordering Syria, on December 31, 2019. - Several thousand Iraqi protesters attacked the US embassy in Baghdad on today, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. AFP via Getty Images Baghdad embassy attack: in pictures Hashd al-Shaabi (paramilitary forces) fighters set fire on the U.S. Embassy wall to condemn air strikes on their bases, in Baghdad, Iraq December 31, 2019. REUTERS Baghdad embassy attack: in pictures Hashd al-Shaabi (paramilitary forces) fighters set fire on the U.S. Embassy wall to condemn air strikes on their bases, in Baghdad, Iraq December 31, 2019. REUTERS Baghdad embassy attack: in pictures Protesters and militia fighters gather to condemn air strikes on bases belonging to Hashd al-Shaabi (paramilitary forces), outside the main gate of the U.S. Embassy in Baghdad, Iraq December 31, 2019. REUTERS Baghdad embassy attack: in pictures Several thousand Iraqi protesters, waving flags in support of Iraq's Hezbollah movement and the Hashed al-Shaabi, a mostly Shiite network of local armed groups trained and armed by powerful neighbour Iran, demonstrate outside the US embassy in Baghdad on December 31, 2019, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. - It was the first time in years protesters have been able to reach the US embassy in the Iraqi capital, which is sheltered behind a series of checkpoints in the high-security Green Zone. AFP via Getty Images Baghdad embassy attack: in pictures Several thousand Iraqi protesters, waving national flags and banners of the Hashed al-Shaabi, a mostly Shiite network of local armed groups trained and armed by powerful neighbour Iran, demonstrate outside the US embassy in Baghdad on December 31, 2019, breaching its outer wall and chanting "Death to America!" in anger over weekend air strikes that killed pro-Iran fighters. - It was the first time in years protesters have been able to reach the US embassy in the Iraqi capital, which is sheltered behind a series of checkpoints in the high-security Green Zone. AFP via Getty Images

Two Apache helicopters later carried out a “show of force” over the embassy, firing off flares as they flew overhead. Officials said scores more US Marines had also been sent to the mission.

Reuters, citing unnamed Iraqi foreign ministry officials, claimed US embassy staff and the US ambassador in Baghdad had been evacuated, but a US official denied the report, saying no American personnel had been forced to leave the facility.

“CJTF-OIR is closely monitoring the current situation of the protests at the US embassy in Baghdad. No evacuations of any embassy personnel have occurred,” the US armed forces, using the acronym for US-led forces in Iraq said in a statement. “CJTF-OIR personnel in Baghdad are taking the appropriate force protection measures to ensure their safety.”

The fallout from the American airstrikes marked a nadir in US-Iraqi relations, with Baghdad officials condemning the American airstrikes and some, including the defence minister and a national security adviser, reportedly joining in the demonstrations outside the embassy Tuesday. The US says it launched the airstrikes in response to an attack on a military base in northern Iraq which left an American contractor dead on Friday.

“There will be a constituency of Iraqis who would like to see America get one on the nose,” Fanar Haddad, an Iraqi scholar at the Middle East Institute of the National University of Singapore, told The Independent. “Whether we like it or not, some of [the Iranian-backed figures] are pillars of the establishment.”

US officials have been jumpy about diplomatic security since a 1983 car bomb destroyed the US mission in Beirut and bombs struck US embassies in Kenya and Tanzania in 1998. US embassy officials in Lebanon issued a warning about demonstrations planned for the American mission in Beirut on Tuesday.

The base attack, US airstrikes, and embassy attack has added a new element of uncertainty and volatility to Iraq’s already potent mix of threats which include Isis elements, Iranian-backed militias, and a popular protest movement seeking to bring down the government. It also may signal a tactical setback for the US which is vying with Iran for influence as Iraqi factions attempt to form a new government.

It remains unclear how the protesters managed to get into the Green Zone and so close to the US embassy. Though security around the highly sensitive area – which includes government offices, the United Nations compound and many foreign diplomatic missions – has been loosened in recent months, the Iraqi forces protecting it have repeatedly barred anti-government demonstrators from getting inside. Iraq’s caretaker prime minister, Adel Abdel Mahdi, called on Iraqis to vacate the embassy area immediately.

“This is a reminder that any attack or harassment to embassies and foreign representation [buildings] are acts that will be firmly prevented by security forces and severely punished by the law,” he wrote in a post on his Facebook page.

But Iraqi officials also issued a stern message to the US over Sunday’s airstrikes, with calls increasing among all the countries’ political factions for a hasty departure of American troops from the country. “The Iraqi government condemns this act and considers it a violation of Iraq’s sovereignty,” said a statement issued by Iraq’s National Security Council.

“This is a serious violation of the rules of engagement,” said the statement. “Tens of members of the Iraqi forces have fallen or injured as a result of this attack. The US forces relied on their own conclusions and political priorities, not the priorities as assessed by the government and people of Iraq.”

The day’s chaos began as Iraqi supporters of Kataib Hezbollah, some holding coffins of those killed in the airstrikes, began gathering outside at the US embassy in Baghdad. Demonstrators draped the yellow flags belonging to the militia on the walls of the embassy.

Others were carrying flags of the Popular Mobilisation Forces, a collection of volunteer armed groups that sprang up in 2014 to fight against Isis and have since grown to become part of the Iraqi security forces.

Some of the protesters began to set up a protest encampment, emulating anti-Iranian protesters who have been demonstrating in Baghdad’s Tahrir Square against the current government for weeks.

Violence came quickly. Al Jazeera television broadcast footage showing protesters attempting to break the windows of the embassy. Embassy security personnel were spotted retreating to the inside of the embassy as the protesters hurled water bottles and smashed security cameras outside the embassy, which measures some 420,000 sq meters and is considered the largest and most expensive American diplomatic mission abroad.

Fires then ensued, with US military personnel spotted on the roof of one of the buildings that make up the embassy.

​It appeared the violence could be escalating beyond what organisers of the funeral march had intended as a man on a loudspeaker urged protesters not to enter the compound. ”The message was delivered,” a man declared over a megaphone, the Associated Press reported.

The confrontation in Baghdad came after US warplanes struck five facilities operated by Kataib Hezbollah in western Iraq and eastern Syria on Sunday, two days after a barrage of missiles hit an Iraqi base, killing one US military contractor and injuring several US troops. No one claimed responsibility for the attack on the base which hosted Americans, but security experts mostly agree it was likely staged by one of several militias linked to Iran.

Among the 40 or so armed groups that make up the Popular Mobilisation Forces, Kataib Hezbollah is widely considered very close to Iran, and is believed to have operational ties to the Quds Force, the clandestine overseas arm of the Iranian Revolutionary Guard which is overseen by Qassem Suleimani, an Iranian military commander.

While the US considers Kataib Hezbollah a terrorist group, it is viewed as a legitimate part of the security forces by some Iraqis.