President Donald Trump on Tuesday blamed Iran for planning an attack on the U.S. Embassy in Iraq and promised to hold Tehran “fully responsible.”

“Iran killed an American contractor, wounding many. We strongly responded, and always will,” the president wrote on Twitter. “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 added.

Later Tuesday, Trump amplified his harsh words for Iran. “This is not a Warning, it is a Threat,” the president tweeted. “Happy New Year!”

Iran denied it was behind the violent protests and warned against any retaliation. “American officials have the astounding audacity of attributing to Iran the protests of the Iraqi people against (Washington’s) savage killing of at least 25 Iraqis …,” Iranian Foreign Ministry spokesman Abbas Mousavi said in a statement carried on a ministry website.

Trump’s tweets came after dozens of angry Iraqi Shiite militia supporters stormed the U.S. Embassy compound in Baghdad and set fire to a reception area on the grounds earlier in the day.

The Iraqi supporters, many dressed in military apparel, pushed into the compound using cars to break through its gate. The protesters hung a poster on the wall saying, “America is an aggressor.”

Though the mob stopped short of entering the main building of the compound, many shouted “Down, Down, USA!” and flung water and rocks over the embassy walls. About 30 Iraqi soldiers in armored vehicles arrived near the embassy hours after the violence, according to an Associated Press report.

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The attack followed deadly U.S. airstrikes on Sunday that killed 25 fighters of the Iran-backed militia in Iraq, carried out in retaliation for last week’s killing of an American contractor in a rocket attack on an Iraqi military base that American officials blamed on the militia.

“The President is in close touch with his national security team and is receiving regular updates,” said White House Press Secretary Stephanie Grisham. “As the President said, Iran is orchestrating this attack and they will be held fully responsible. It will be the President’s choice how and when we respond to their escalation.”

Trump also spoke with Iraqi Prime Minister Adel Abd al-Mahdi and emphasized the need to protect U.S. personnel and facilities, the White House said in a statement later Tuesday.

Secretary of Defense Mark Esper said the U.S. would be sending additional troops to the compound to assist in its protection.

“We are sending additional forces to support our personnel at the Embassy. As in all countries, we rely on host nation forces to assist in the protection of our personnel in country, and we call on the Government of Iraq to fulfill its international responsibilities to do so,” he said in a statement.

“We have taken appropriate force protection actions to ensure the safety of American citizens, military personnel and diplomats in country, and to ensure our right of self-defense,” Esper added.

The U.S. strikes over the weekend were the latest to target the Iraqi state-sanctioned and Iranian-backed militia and appear to mark the start of a more fierce power struggle between the U.S. and Iran throughout the Middle East.

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While Secretary of State Mike Pompeo said Sunday’s strikes were designed to convey that the White House will not turn a blind eye to actions taken by Iran that jeopardize American lives, the Iraqi government deemed the attack a “flagrant violation” of its sovereignty and said it will reexamine its relationship with the U.S.-led coalition.

— CNBC’s Amanda Macias and Reuters contributed to this report.