BAGHDAD — Dozens of Iraqi Shiite militiamen and their supporters broke into the U.S. Embassy compound in Baghdad on Tuesday, smashing a main door and setting fire to a reception area, angered over deadly U.S. airstrikes targeting the Iran-backed militia. U.S. guards fired tear gas and palls of smoke rose over the grounds.

An Associated Press reporter at the scene saw flames rising from inside the compound and at least three U.S. soldiers on the roof of the main embassy building. There was a fire at the reception area near the compound’s parking lot but it was unclear what had started it. A man on a loudspeaker urged the mob not to enter the compound, saying: “The message was delivered.”

There were no reports of casualties, but the unprecedented breach was one of the worst attacks on the embassy in recent memory. It followed deadly U.S. airstrikes on Sunday that killed 25 fighters of the Iran-backed militia in Iraq, the Kataeb Hezbollah. The U.S. military said the airstrikes were in retaliation for last week’s killing of an American contractor in a rocket attack on an Iraqi military base that it had blamed on the militia.

President Donald Trump blamed Iran for the embassy breach and called on Iraq to protect the diplomatic mission.

“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!” he tweeted from his estate in Palm Beach, Florida.

By early evening, protesters had set up tents outside the embassy where they said they intended to stage a sit-in. Dozens of yellow flags belonging to Iran-backed Shiite militias fluttered atop the reception area and were plastered along the embassy’s concrete wall along with anti-U.S. graffiti.

The developments represent a major downturn in Iraq-U.S. relations that could further undermine U.S. influence in the region and also weaken Washington’s hand in its maximum pressure campaign against Iran.

Iraq has long struggled to balance its ties with the U.S. and Iran, both allies of the Iraqi government. But the government’s angry reaction to the U.S. airstrikes and its apparent decision not to prevent the protesters from reaching the embassy signaled a sharp deterioration of U.S.-Iraq relations.

Iraqi security forces made no effort to stop the protesters as they marched to the heavily-fortified Green Zone after a funeral held for those killed in the U.S. airstrikes, letting them pass through a security checkpoint leading to the area.

The mob of marchers, many of them in militia uniforms, shouted “Down, Down USA!” and “Death to America” and “Death to Israel” outside the compound, hurling water and stones over its walls. The mob set fire to three trailers used by security guards along the wall. AP journalists saw some try to scale the walls.

Others then smashed the gates used by cars to enter and dozens pushed into the compound. The protesters stopped in a corridor after about 5 meters (16 feet), and were only about 200 meters away from the main building. Half a dozen U.S. soldiers were seen on the roof of the main building, their guns were pointed at the protesters. Smoke from the tear gas rose in the area.

The protesters raised yellow militia flags and taunted the embassy’s security staff who remained behind the glass windows in the gates’ reception area. They hung a poster on the wall declaring, “America is an aggressor” and sprayed graffiti on the wall and windows reading, “Closed in the name of the resistance.”

“This is a victory in retaliation to the American airstrike. This is the initial retaliation, God willing, there will be more,” said Mahmoud, a fighter with the Imam Ali Brigades who was carrying a black bag filled with electricity cables that he said he took from the embassy’s reception area.

A video obtained by the AP showed militiamen trashing the embassy’s reception area and also taking away paperwork.

The Embassy, on its Facebook page, urged American citizens not to approach the compound and “to review their personal security and emergency preparedness.”

An Iraqi employee at the embassy told the AP that the embassy’s security team had evacuated some local staff from a rear gate while others left by helicopters as the rest remained inside “safe” areas within the embassy. The employee spoke on condition of anonymity because of not being authorized to speak to journalists.

Multiple officials told the AP that the U.S. ambassador was already traveling outside Iraq before the attack on the compound.

Some commanders of militia factions loyal to Iran had joined the protesters. Among them was Qais al-Khizali, the head of one of the most powerful Iranian-backed Shiite militias in Iraq who is on a U.S. terror list, and Hadi al-Amiri, the head of the state-sanctioned paramilitary Popular Mobilization Units, the umbrella group for the Iran-backed militias.

Jaafar al-Husseini, a spokesman for Kataeb Hezbollah, said the protesters had no intention to storm the embassy, only to send a message. He told the AP that the sit-in outside the embassy will continue “until American troops leave Iraq and the embassy is closed.”

At least three protesters appeared to have difficulties breathing from tear gas. No one was immediately reported hurt in the rampage.

Yassine al-Yasseri, Iraq’s interior minister, also appeared outside the embassy at one point and walked around to inspect the scene. He told the AP that the prime minister had warned the U.S. strikes on the Shiite militiamen would have serious consequences.

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

Seven armored vehicles with about 30 Iraqi soldiers arrived near the embassy hours after the violence erupted, deploying near the embassy walls but not close to the breached area.

There was no immediate comment from the Pentagon and the State Department.

The U.S. airstrikes — the largest targeting an Iraqi state-sanctioned militia in recent years — and the subsequent calls by the militia for retaliation, represent a new escalation in the proxy war between the U.S. and Iran playing out in the Middle East.

U.S. Secretary of State Mike Pompeo said Sunday’s strikes send the message that the U.S. will not tolerate actions by Iran that jeopardize American lives.

The U.S. attack also outraged the Iraqi government, which said it will reconsider its relationship with the U.S.-led coalition — the first time it has said it will do so since an agreement was struck to keep some U.S. troops in the country. It called the attack a “flagrant violation” of its sovereignty.

In a partly televised meeting Monday, Iraq’s caretaker Prime Minister Adel Abdul-Mahdi told Cabinet members that he had tried to stop the U.S. operation “but there was insistence” from American officials. He declared three days of mourning for those killed in the U.S. strikes, starting Tuesday.

The U.S. military said “precision defensive strikes” were conducted against five sites of Kataeb Hezbollah, or Hezbollah Brigades in Iraq and Syria. The group, which is a separate force from the Lebanese militant group Hezbollah, operates under the umbrella of the state-sanctioned militias known collectively as the Popular Mobilization Forces. Many of them are supported by Iran.