Demonstrators shouting, "Death to America!" smashed their way into the U.S. Embassy compound in Baghdad on Tuesday as protests intensified after U.S. airstrikes killed 25 fighters of an Iran-backed militia in Iraq.

No deaths or serious injuries were immediately reported at the protest. The Pentagon said it rushed reinforcements to the embassy, and Army Apache AH-64 attack helicopters swooped over the complex.

By early evening Tuesday, the mob had retreated from the compound but set up several tents outside for 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.

Iraqi Prime Minister Adel Abdul Mahdi, who condemned the airstrikes, had urged militiamen and their supporters to leave the embassy area and halt the violence.

"Any aggression or harassment of foreign embassies and representations is an act that will be strictly prohibited by the security forces and will be punished by law with the most severe penalties," Mahdi warned in a statement.

Embassy attack: Trump blames Iran for insurgents storming US Embassy in Iraq

Protesters had barged through a door and set fire to a reception area. Smoke and flames rose from the area as a man on a loudspeaker urged the mob not to enter the compound, saying, “The message was delivered.”

U.S. guards had fired tear gas to drive back the crowd, and soldiers manned the roof of the main building, their guns pointed at the protesters. Iraqi forces that guard the usually safe Green Zone where the embassy is located did little to stop the chaos.

The State Department said all embassy personnel were safe and that no evacuation was planned.

The U.S. reinforcements included about 100 Marines, a U.S. military official who was not authorized to speak publicly told USA TODAY.

“We rely on host nation forces to assist in the protection of our personnel in country," U.S. Defense Secretary Mark Esper said in a statement. "We call on the government of Iraq to fulfill its international responsibilities to do so."

Secretary of State Mike Pompeo defended the airstrikes as a "decisive response" to a rocket attack on an Iraqi military base that killed an American contractor last week.

President Donald Trump, on Twitter, accused Iran of orchestrating the embassy attack.

"Iran killed an American contractor, wounding many. We strongly responded, and always will," Trump tweeted. "They will be held fully responsible. In addition, we expect Iraq to use its forces to protect the Embassy, and so notified!"

Trump spoke Tuesday with Mahdi, the White House announced. The two leaders discussed regional security issues, and Trump emphasized the need to protect U.S. personnel and facilities in Iraq.

Late Tuesday afternoon, Trump tweeted "Many of our great Warfighters, together with the most lethal military equipment in the world, was immediately rushed to the site. Thank you to the President & Prime Minister of Iraq for their rapid response upon request....

"Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities," Trump said. "They will pay a very BIG PRICE! This is not a Warning, it is a Threat."

In another tweet, Trump called Tuesday's incident the "Anti-Benghazi."

Pompeo said he spoke to Mahdi and President Barham Salih separately by phone, making clear the United States will protect and defend Americans working to "support a sovereign and independent Iraq." Pompeo said the Iraqi leaders guaranteed the safety and security of U.S. personnel and property.

Mahdi said he tried to stop the airstrikes, “but there was insistence” from U.S. officials. He announced three days of mourning starting Tuesday out of respect for "the lives of the martyrs of our armed forces ... following the unjust assault they were subjected to."

James Piazza, a Penn State political science professor specializing in the Islamic world, said the airstrikes and the storming of the U.S. Embassy underscore the difficult position the Iraqi government faces.

"Mahdi made a statement condemning the attacks, but it does not seem to be making an impression on the U.S. and on the Iraq people," Piazza told USA TODAY. "The situation makes the Iraqi government seem feeble."

Kirsten Fontenrose, former senior director for Gulf affairs at the National Security Council, dismissed the embassy protest as a "page out of the Iranian clerical regime's favorite playbook."

Fontenrose said the embassy attack was an attempt to recreate the seizure of the U.S. Embassy in Iran 40 years ago. Dozens of Americans were held hostage for more than a year.

"It helps reignite the revolutionary zeal that is critical to their regime survival but is lagging in their own country, where the population has tired of that rhetoric," she said.

The Pentagon, in defending the airstrikes, cited repeated attacks by the Kataeb Hezbollah militia on Iraqi bases that host Operation Inherent Resolve coalition forces. The "defensive" airstrikes targeted three sites in Iraq and two in Syria that included weapon storage facilities and command and control locations, the military said.

Kataeb Hezbollah operates under the umbrella of state-sanctioned militias known collectively as the Popular Mobilization Forces. Most of them are supported by Iran. The Iranian-backed Iraqi militia vowed Monday to retaliate for the U.S. military strikes.

Pentagon:US conducts strikes on militia behind attack that killed American

The Washington-based National Iranian American Council issued a statement accusing the Trump administration of conducting a "reckless and needless 'maximum pressure' campaign" that threatens to turn Iraq into a battlefield between the United States and Iran.

"Avoiding this scenario requires a broader rethinking of the maximum pressure policy away from mindless saber rattling to one that opens channels of dialogue with Tehran and explores potential avenues for renewed cooperation to help stabilize Iraq," NIAC Senior Research Analyst Sina Toossi said.

Contributing: Tom Vanden Brook and Doyle Rice, USA TODAY; The Associated Press