President Trump warned Iran that the country “will pay a very BIG PRICE!” after the break-in at the U.S. Embassy compound in Baghdad on Tuesday.

“Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities,” Trump tweeted Tuesday afternoon. “They will pay a very BIG PRICE! This is not a Warning, it is a Threat. Happy New Year!”

Trump has blamed Iran for the embassy breach in which dozens of Iraqi Shiite militiamen and their supporters broke into the U.S. Embassy, smashing a main door and setting fire to a reception area in one of the worst attacks on the embassy in recent years.

There were no reports of casualties. The State Department said all American personnel were safe and that there were no plans to evacuate the embassy. The government planned to send more troops to protect the compound.

US PRESSES IRAQ TO PROTECT AMERICAN PERSONNEL AFTER EMBASSY ATTACK: ‘THERE WILL BE NO BENGHAZIS’

A U.S. defense official told Fox News that 100 Marines had been sent to the embassy to bolster security. A U.S. Apache helicopter gunship also flew over the embassy and dropped flares in a “show of force,” attempting to disperse the crowd. Defense Secretary Mark Esper later confirmed additional forces were being sent, as the military released images of those forces deploying from Kuwait.

The breach followed 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 strikes were in retaliation for last week's killing of an American contractor in a rocket attack on an Iraqi military base that the U.S. blamed on the militia.

“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!" Trump tweeted earlier on Tuesday from his estate in Palm Beach, Fla.

Trump, who is spending the holiday week at his Florida home, is in “close touch” and receiving regular updates from his national security team, said White House press secretary Stephanie Grisham. She echoed the sentiment contained in Trump’s tweet earlier Tuesday.

“As the president said, Iran is orchestrating this attack, and they will be held fully responsible,” Grisham said in an emailed statement. “It will be the president’s choice how and when we respond to their escalation.”

CLICK HERE FOR THE FOX NEWS APP

The developments also represent a major downturn in Iraq-U.S. relations that could further undermine U.S. influence in the region and American troops in Iraq and weaken Washington's hand in its 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.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.