U.S. President Donald Trump has warned Iran against any attack on U.S. military or diplomatic personnel in Iraq, days after suspected Iran-backed Iraqi militia launched a barrage of rockets at the U.S. Embassy in Baghdad’s heavily fortified Green Zone.

The warning, issued on Twitter on December 23, came after top U.S. national-security officials met to prepare a range of options to propose to the president in order to deter any attack on U.S. interests in Iraq.

The so-called principals committee group, including acting Defense Secretary Chris Miller, Secretary of State Mike Pompeo, and national-security adviser Robert O'Brien met at the White House, Reuters reported, citing an anonymous official.

For weeks, U.S. officials have suggested Iran or allied Iraqi militia could carry out retaliatory attacks to mark the first anniversary of the U.S. drone strike that killed Iran’s top general, Qasem Soleimani, and Iraqi militia leaders outside Baghdad's airport on January 3.

The aim of the White House meeting was "to develop the right set of options that we could present to the president to make sure that we deter the Iranians and Shi'a militias in Iraq from conducting attacks on our personnel,” a senior administration official told Reuters.

Following the meeting, Trump took to Twitter to comment on a hail of rockets that targeted Baghdad’s heavily fortified Green Zone on December 20, causing minor damage to the U.S. Embassy compound and residential areas in the international zone.

“Our embassy in Baghdad got hit Sunday by several rockets. Three rockets failed to launch. Guess where they were from: IRAN,” Trump wrote above a picture claiming to show rockets from Iran.

“Now we hear chatter of additional attacks against Americans in Iraq. Some friendly health advice to Iran: If one American is killed, I will hold Iran responsible. Think it over,” Trump wrote, repeating a redline over any American casualties.

Iranian Condemnation

Iran’s foreign minister dismissed Trump’s allegations that Iran was behind the recent rocket attack.

“Putting your own citizens at risk abroad won't divert attention from catastrophic failures at home,” Foreign Minister Mohammad Javad Zarif said on Twitter.

Foreign Ministry spokesman Saeed Khatibzadeh described Trump’s accusations as “baseless” and said that Iran has repeatedly condemned attacks on diplomatic and residential sites.

Khatibzadeh was quoted by the official government news agency IRNA as saying that Tehran considers the U.S. responsible for the consequences of "any unwise move" in the current condition.

"In this specific issue, the fingers are pointed at the U.S. and its partners and allies in the region that are trying to increase tension," he said.

The U.S. military's Central Command (CENTCOM) said in a statement that the rocket attack in Iraq was “almost certainly” carried out by an “Iranian-backed rogue militia group.”

“While this 21 rocket attack caused no U.S. injuries or casualties, the attack did damage buildings in the U.S. Embassy compound, and was clearly NOT intended to avoid casualties,” the statement said.

Following the December 20 attack, an Iraqi military statement said “an outlawed group” launched eight rockets at the Green Zone, the location of embassies and government buildings.

Most of the rockets landed near an empty residential complex and checkpoint, injuring one Iraqi security person.

Although no Americans were killed or injured by the rockets, the attack and Trump’s threat underscore a highly combustible situation in Iraq that could quickly spiral out of control.

A rocket attack blamed on Iran-backed militia in December 2019 killed a U.S. defense contractor and wounded several U.S. and Iraqi soldiers at a military base in the northern Iraqi city of Kirkuk, touching off a cycle of escalation that led to Soleimani's killing and Iran launching retaliatory ballistic missiles at Iraqi bases housing U.S. troops, bringing the two rivals to the brink of full-fledged war.

In a new show of force directed at Iran around the anniversary of Soleimani’s killing, a U.S. nuclear submarine carrying 154 Tomahawk cruise missiles crossed the Strait of Hormuz on December 21.

The aircraft carrier USS Nimitz has been patrolling Gulf waters since late November, and two American B-52 bombers recently overflew the region in a demonstration of strength aimed at Iran.

"My assessment is we are in a very good position and we'll be prepared for anything the Iranians or their proxies acting for them might choose to do," General Kenneth McKenzie, the U.S. commander for the Middle East, told journalists on December 20.

U.S. officials have blamed Iran-backed Iraqi militia for carrying out a string of attacks on U.S. interests in Iraq this year, prompting Washington to threaten a diplomatic and military withdrawal from the country.

The Trump administration in November ordered a reduction of U.S. troops in Iraq from 3,000 to 2,500 by mid-January.

Several Iraq militia groups in October announced a brief suspension of attacks on U.S. interests on condition that a timetable would be presented for U.S. forces to leave Iraq. That truce came to an end on November 18 with a rocket strike on the U.S. Embassy.

Meanwhile, the United States said in early December it was partially withdrawing some staff from its embassy in response to rising tensions.

U.S. officials say the temporary staff reduction was in response to possible threats around the anniversary of Soleimani's killing and that of leading Iraqi paramilitary figure Abu Mahdi al-Muhandis.

Aggravating the situation, tensions spiked again across the region following assassination of Iranian nuclear scientist Mohsen Fakhrizadeh near Tehran in late November. Iran has blamed Israel and, indirectly, the United States, raising the possibility that Iran or one of its regional proxies will retaliate.

The developments in Iraq come as Trump ramps up pressure on Iran ahead of a transition to President-elect Joe Biden, who has said he will try to revive diplomacy with Iran upon entering the White House in January.

Biden is expected to try to rejoin the Iran nuclear accord that Trump quit in 2018 and work with allies to strengthen its terms, if Tehran resumes compliance.

Western diplomats and media reports have suggested Iran has told allied Iraqi militia groups to avoid provoking the United States in the final weeks of the Trump administration out of concern the situation could escalate before a more dovish Biden administration comes to power.

But there are also questions about how much direct operational control Iran really maintains over an array of Iraqi militia groups and proxies across the region, raising the prospect of accidents and miscalculations.

"I do believe we remain in a period of heightened risk," McKenzie, the head of U.S. Central Command, told ABC News on December 22. "I would just emphasize this key point: We're not looking to escalate ourselves. We're not looking for war with Iran, I really want to emphasize that."

"It is my belief that Iran doesn't want a war with the United States right now," he added.

With reporting by AFP and Reuters.