Protesters on Tuesday stormed the U.S. Embassy in Baghdad after the U.S. launched airstrikes on an Iranian-backed militia in the country over the weekend that killed 25 people.

The U.S. attacks were in response to a Friday missile attack on an Iraqi base that killed a U.S. contractor. The missile attack was blamed on the Iranian-backed militia Kataib Hezbollah, which has denied it was responsible.

The attacks on the embassy represent the worst political crisis for the United States in Iraq in years. Iraqi forces apparently did nothing to stop protestors from moving through security checkpoints in the Green Zone holding the U.S. embassy, though Iraq's government later warned of punishment for damage to the embassy.

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The U.S. airstrikes on Kataib Hezbollah had been criticized by Iraqi government officials, who had warned they would provoke a response.

The attack was described by the AP as one of the worst embassy attacks in recent history.

Burning the outer fence of the US embassy pic.twitter.com/wAeLjjQMy4 — mustafa saadoon (@SaadoonMustafa) December 31, 2019

In response, President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE accused Iran of being behind them, vowed that Tehran would be held responsible and called on Iraqi forces to help protect the U.S. embassy.

"They will be held fully responsible," he tweeted. "In addition, we expect Iraq to use its forces to protect the Embassy, and so notified!"

Iran killed an American contractor, wounding many. We strongly responded, and always will. Now Iran is orchestrating an attack on the U.S. 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! — Donald J. Trump (@realDonaldTrump) December 31, 2019

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Defense Secretary Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE said in a statement that the department is working with the State Department to make sure the embassy and personnel in Baghdad are safe. He also called on Iraq's government to protect U.S. personnel.

"We have taken appropriate force protection actions to ensure the safety of American citizens, military personnel and diplomats ... and to ensure our right of self-defense," Esper said in a statement obtained by The Hill. "We are sending additional forces to support our personnel at the Embassy."

A U.S. official told Bloomberg that the Pentagon sent two helicopters to fly over the embassy as a show of force, and about 100 Marines will be sent to the embassy.

The State Department did not immediately return requests to comment.

Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE talked to Iraqi Prime Minister Adil Abdul Mahdi and President Barham Salih separately by phone from his office in Washington, D.C., State Department spokeswoman Morgan Ortagus said in a statement.

“The secretary made clear the United States will protect and defend its people, who are there to support a sovereign and independent Iraq,” she said. “Both Abdul-Mahdi and Salih assured the secretary that they took seriously their responsibility for and would guarantee the safety and security of U.S. personnel and property.”

Iran's government has long been seen as having a heavy influence in Iraq and the attacks on the embassy are likely to spotlight Tehran's efforts to flex its power in Baghdad.

Yassine al-Yasseri, Iraq's interior minister, was on the scene and told the AP that the prime minister had cautioned that the U.S. strikes over the weekend would cause a reaction.

“This is one of the implications,” al-Yasseri said. “This is a problem and is embarrassing to the government.”

Protesters smashed a main door to the compound and set a fire in the reception area, leading to the disposal of tear gas and gunfire, The Associated Press reported.

Thousands of demonstrators and supporters of the militia protested outside while chanting "Death to America" to express anger over the attacks.

An AP reporter on the scene reported that flames were rising inside the compound, and at least three U.S. soldiers were on the roof of the main building.

U.S. diplomats and embassy staffers were brought to a fortified safe room, two of them told The Washington Post, adding that they feel secure.

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The protesters, including some commanders of militia factions and some in militia uniform, pushed through a gate intended for cars. They at first stopped in a corridor a few hundred feet away from the building, while U.S. soldiers on top pointed their firearms at the protesters.

The demonstrators hung a sign saying "America is the aggressor" and shouted, "Down, Down USA!"

The demonstrators reportedly attended funerals for the fighters killed in the strikes over the weekend in Baghdad before marching to the U.S. Embassy. They threw water bottles and stones over the walls, raised yellow militia flags, set up tents around the embassy and sprayed graffiti in support of the militia.

Three trailers used by security guards at the embassy were also set on fire, but no one was immediately reported injured.

Embassy security utilized tear gas to try to prevent the crowd from moving toward the main building, an Iraqi security source told NBC News.

Live bullets and tear gas injured at least 10 protesters, according to the Popular Mobilization Forces, a group for the militias recognized in Iraq, NBC News reported.

The U.S. says Kataib Hezbollah is responsible for 11 attacks on U.S.-led coalition bases in the past two months, most recently one on Friday that killed a U.S. contractor and injured four U.S. military members.

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A spokesman for Kataib Hizbollah said the group plans to stake out the street in front of the embassy to push U.S. officials to leave Iraq.

“We will not leave these tents until the embassy and the ambassador leave Iraq,” Mohammed Muhi said, according to the Times.

Updated at 10:11 a.m.