(CNN) President Donald Trump threatened Iran Tuesday, warning the country's leaders they will be held responsible for any death or destruction after protestors attacked the US embassy in Baghdad.

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

And later Tuesday, while speaking with reporters as he entered a New Year's Eve gala at Mar-a-Lago, the President said he doesn't want war but that if it comes to conflict, Iran wouldn't last long.

"I don't think that would be a good idea for Iran," Trump said, adding later that a potential conflict "would go very quickly."

As night fell, protestors set up tents near the embassy, suggesting another day of upheaval lies ahead. Iraqi counterterrorism forces were deployed around the perimeter of the embassy, while the Pentagon announced additional Marines would be sent to Baghdad and more troops would be deployed to the Middle East in response to the embassy crisis.

'It's going to be tense'

Trump stressed safety in a set of tweets and drew a direct comparison to the 2012 attack in Benghazi, Libya, that left four Americans dead, even as a senior administration official told CNN the White House is "very concerned" about the potential for further escalation come New Year's Day morning.

"Tomorrow, during the daytime, it's going to be tense," the official said.

Another senior administration official said the administration is "not going to assume it's an escalation, but also is not going to be unaware" of the potential for further deterioration with Iran, adding that "it's a complicated situation."

The administration hopes everything will calm down and go back to normal, the official said, but is prepared for all contingencies and is keeping a close eye on the situation.

Protesters at the US embassy in Baghdad on Tuesday.

This official added that Trump emphasized in a call with the Iraqi prime minister that Iraq has a responsibility to protect the US embassy and personnel.

For now, US officials are closely watching the situation and hopeful that a bolstered Iraqi response and US moves to reinforce security will be enough. Trump insisted in a Tuesday tweet that the embassy was safe.

"The U.S. Embassy in Iraq is, & has been for hours, SAFE! Many of our great Warfighters, together with the most lethal military equipment in the world, was immediately rushed to the site," Trump tweeted. "The Anti-Benghazi!" he added in a separate post.

The US military is expected to soon deploy additional military forces to the Middle East in response to the situation, a US defense official told CNN, on top of about 100 additional Marines from a crisis response task force based in Kuwait that arrived in Baghdad today to bolster embassy security.

The number of troops could be as large as "a brigade type element" or a few thousand additional US soldiers drawn mostly from the 82nd Airborne Division in Fort Bragg, North Carolina, according to the official, who said they could arrive in a matter of days at the earliest.

The attacks on the embassy are a response to US airstrikes on Sunday gainst an Iran-backed militia that formally sits under the control of Iraq's prime minister. The airstrikes have caused tension with Baghdad, where officials accused the US of attacking without sufficient evidence, violating Iraqi sovereignty and threatening the country's security.

Iran pushes back

While Trump, US officials and lawmakers placed blame for the protests squarely on Iran, a foreign ministry spokesman in Tehran vehemently denied the allegations.

In a Tuesday statement, Abbas Mousavi called the US allegation "obscenity" while "Iraqi people are overwhelmed with bloodshed [in which] at least 25 people died," a reference to the death toll of US airstrikes on the militia, Kataib Hezbollah, in Iraq on Sunday.

"How and on what basis do you expect the Iraqi people to remain silent on all these crimes?" Mousavi said, adding that the United States has "ignored" Iraqi independence and called on Washington to "reconsider its destructive policies in the region."

Both the White House and militia representatives signaled Tuesday that the situation could escalate, while US officials stressed their readiness to protect American personnel, flying Apache helicopters over the embassy in a show of force and announcing the military reinforcements.

White House spokeswoman Stephanie Grisham cast the protests as an Iranian "escalation," saying that "it will be the President's choice how and when we respond."

VIDEO: AH-64 Apaches protect @USEmbBaghdad. 🚁"We have taken appropriate force protection actions to ensure the safety of American citizens...and to ensure our right of self-defense. We are sending additional forces to support our personnel at the Embassy."~@EsperDoD 🇺🇸🇮🇶 pic.twitter.com/amABHBAOcL — OIR Spokesman Col. Myles B. Caggins III (@OIRSpox) December 31, 2019

Meanwhile, a spokesman for the Iran-backed militia, Kataib Hezbollah, told CNN that the embassy protests were only a "first step."

"The American administration should understand the following: the first step was to protest near the American embassy, we are waiting their reactions to determine the second step," Mohamad Mouhiye said, calling for the US to shut down the embassy and withdraw from Iraq.

"First, close the doors of the evil embassy, which we consider a spy building and an operations room to administer and sabotage Iraq's wellbeing," Mouhiye said. "We also call on the US to withdraw their military forces which are in Iraq illegally," he said.

"This protest was a stand against US political dominance," he said.

No evacuation plans

The embassy, in Baghdad's green zone, has been put under lockdown, an embassy spokesperson told CNN, but a State Department spokesperson insisted "there has been no breach." Secretary of State Mike Pompeo told Fox News Tuesday evening that there are currently no plans to evacuate the embassy or pull out US troops.

US ambassador to Iraq Matthew Tueller has been on vacation for more than a week, but a State Department spokesperson said Tueller is returning to the embassy.

Defense Secretary Mark Esper said the Pentagon is working closely with the State Department to ensure the safety of the embassy and staff in Baghdad. He also reminded Iraq of its obligations under international law.

Iraqi protesters set a sentry box ablaze in front of the US embassy in Baghdad on Tuesday.

"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 said. "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."

It is not clear how many people are inside the embassy, but it is the US' largest diplomatic mission in the world, with around 16,000 staff. The compound covers around 100 acres, around the same size as the Vatican City.

The pro-Iranian demonstrators were mostly from Iraq's Popular Mobilization Units (PMU), a coalition of predominantly Shiite militias. Formed in 2014 to fight ISIS, the PMU were recognized under a 2016 Iraqi law as an independent military force that answers directly to the prime minister.

Despite the Iraqi law, most observers see many of those groups, including Kataib Hezbollah, as maintaining strong links to Iran and its security forces, from which they receive funding and other support, as well as some direction.

Three leaders of powerful militia groups were also seen at the protest, including Kataib Hezbollah leader Abu Mahdi al-Muhandis.

The US carried out five airstrikes in Iraq and Syria on Sunday on facilities controlled by Kataib Hezbollah, killing at least 25 people and wounding 51, in the first significant US military response to weeks of deadly rocket attacks by the Iran-backed group on US-Iraqi targets.

US officials said the strikes were carried out with F-15 Strike Eagle fighter planes and targeted weapons storage facilities and command and control locations used by Kataib Hezbollah. The Pentagon said the locations had been used "to plan and execute attacks" on joint US-Iraq forces.

The strikes and ensuing protests come at a time of high tensions between the US and Iran, and have stoked fears of a new proxy war in the Middle East

Washington has tightened the economic squeeze on Tehran this year through its "maximum pressure" campaign, while Iran has responded with what it calls for "maximum resistance," including reducing its compliance to the international nuclear deal

The Trump administration pulled the US out of that deal in May 2018, sparking a campaign of provocation between the two nations that has included attacks by Iran or its proxies on US allies and oil infrastructure and shipping.

Trump blames Iran

Trump confirmed Tuesday that Sunday's airstrikes were a response to a recent attack that killed a US contractor. He blamed Iran both for the contractor's death and Tuesday's attack on the embassy.

"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.

The airstrikes and embassy attack have created new tensions between allies Washington and Baghdad and come at a time of unrest, as mass protests across Iraq challenge the nation's government.

Baghdad warned Monday that its relations with the US were at risk following the strikes. Questions have also been raised as to whether Iraqi forces allowed the protesters to reach the US Embassy, a highly fortified building in an area that is usually restricted.

Iraqi protesters try to storm the US Embassy in Baghdad on Tuesday.

Pompeo spoke with Iraq's embattled Prime Minister Adil Abdul-Mahdi and President Barham Salih on Tuesday, the State Department said in a statement.

"The Secretary made clear the United States will protect and defend its people, who are there to support a sovereign and independent Iraq. Both Abdul-Mahdi and Salih assured the Secretary that they took seriously their responsibility for and would guarantee the safety and security of US personnel and property," State Department spokeswoman Morgan Ortagus said.

Abdul-Mahdi earlier called on protestors to leave the area around the embassy, warning them against acts of aggression.

"We ask everyone to immediately leave these places, and we recall that any aggression or harassment of foreign embassies and representations are an act that will be strictly prohibited by the security forces and will be punished by law with the most severe penalties," Abdul Mahdi said in a statement.

But he also described those killed in the US airstrikes as "martyrs" and supported a funeral for them in the capital's streets.

The headline and story have been updated with additional developments Tuesday.