A US official told CNN that there were no initial reports of any US casualties, but an assessment of the impact of the strikes is underway. A preliminary report from an Iraqi security source indicated there were Iraqi casualties, but Iraqi security officials later told CNN there were no casualties among Iraqi security forces.

White House aides had initially made plans for a possible address to the nation by Trump, according to two officials, but a White House official said that Trump would not speak on Tuesday. The President later tweeted, "All is well! Missiles launched from Iran at two military bases located in Iraq. Assessment of casualties & damages taking place now. So far, so good! We have the most powerful and well equipped military anywhere in the world, by far! I will be making a statement tomorrow morning."

The attack comes days after the US killed top Iranian General Qasem Soleimani in an airstrike in the Iraqi capital of Baghdad. The administration has sought to cast that strike as an attempt to de-escalate tensions with Iran, but Tehran has vowed revenge for the killing, which it says was an "act of war" and "state terrorism."

In a statement, the Islamic Revolutionary Guards Corps, an elite wing of the Iranian military that is also known as the IRGC, said the attacks were "hard revenge" for the death of Soleimani. The IRGC said in the statement that any country housing US troops could be subject to "hostile and aggressive acts" and called on American citizens to demand the government remove US troops from the region.

"To the Great Satan ... we warn that if you repeat your wickedness or take any additional movements or make additional aggression, we will respond with more painful and crushing responses," the statement to the US read.

Jonathan Hoffman, a Pentagon spokesperson, said Tuesday evening that Iran launched more than a dozen missiles at the al-Asad airbase, which houses US troops, and American and coalition forces in the town of Erbil. Hoffman said the Pentagon is assessing the damage done by the attacks.

"In recent days and in response to Iranian threats and actions, the Department of Defense has taken all appropriate measures to safeguard our personnel and partners," Hoffman said in a statement. "These bases have been on high alert due to indications that the Iranian regime planned to attack our forces and interests in the region."

"As we evaluate the situation and our response, we will take all necessary measures to protect and defend U.S. personnel, partners, and allies in the region."

The initial assessment is that the Iranian missiles struck areas of the al-Asad base not populated by Americans, according to a US military official and a senior administration official. Officials have said the US is awaiting daylight to get a full assessment of the results of the strike. Another US military official told CNN the military had enough warning of the launches that they had time to sound alarms. People in harm's way were able to get to safety, according to the official.

At least two ballistic missiles hit separate areas in Erbil, two Kurdish security officials tell CNN. One missile landed inside the perimeter of Erbil International Airport without exploding, the second missile hit an area roughly 20 miles west of Erbil without causing casualties.

US troops were patrolling outside their bases with Iraqi partners in the hours after the strikes, according to a military official who didn't want to be named. Drones and attack helicopters were orbiting bases housing US troops, the official said. CNN saw an AC-130 gunship over central Baghdad, which includes the heavily fortified Green Zone, shortly after the attacks.

US Secretary of State Mike Pompeo, Secretary of Defense Mark Esper and Chairman of the Joint Chiefs of Staff Army Gen. Mark Milley arrived at the White House following the attack and they left later Tuesday night.

Trump visited the base in December 2018 to visit troops after Christmas. Vice President Mike Pence also visited the base in November 2019. The attack follows last week's deadly US drone strike that Trump ordered to kill Soleimani.

Iran claims responsibility

Iranian state TV reported that the IRGC, "has hit U.S. Ain al-Asad airbase in Iraq with tens of missiles." The IRGC warned the US of more "crushing responses in case of new aggression," according to state TV. The IRGC said it will target any regional state that becomes a platform for US aggression, a second banner on state TV read.

Iranian Foreign Minister Mohammad Javad Zarif said on Twitter that the response was meant to be proportionate to the American attack that killed Soleimani.

"Iran took & concluded proportionate measures in self-defense under Article 51 of UN Charter targeting base from which cowardly armed attack against our citizens & senior officials were launched," Zarif tweeted. "We do not seek escalation or war, but will defend ourselves against any aggression."

Esper's office called the office of the Prime Minister of Iraq at around 7 pm ET on Tuesday, according to a diplomatic source. There were difficulties getting through as it was the middle of the night in Baghdad. Eventually the two offices were able to connect and the source says there has been "communication between the US and Iraqi governments at the highest level."

CNN reported earlier on Tuesday that US forces and air-defense missile batteries across the Middle East were placed on high alert overnight Monday to possibly shoot down Iranian drones as intelligence mounted about a threat of an imminent attack against US targets, according to two US officials.

CNN reported on Friday that part of the intelligence that led to the decision to kill Soleimani included threats to al-Asad air base.

A source familiar with the intelligence showed the vehicle mounted rockets, known as Grad trucks, and other military weaponry were moving closer to US interests, particularly the al-Asad air base, CNN reported.

Other targets of concern included the US air base in Qatar and US interests in Kuwait. The source noted on Friday that these threats have existed for several months but that the intelligence indicated growing urgency because of how close the missile trucks were getting to US interests.

The attack came hours after Esper told CNN that the US is not seeking a war with Iran but it is "prepared to finish one."

"We are not looking to start a war with Iran, but we are prepared to finish one," Esper said during an interview with CNN's Christiane Amanpour.

Trump was briefed on the reports of rocket attacks, White House press secretary Stephanie Grisham said.

"We are aware of the reports of attacks on US facilities in Iraq. The President has been briefed and is monitoring the situation closely and consulting with his national security team," Grisham said.

Attack are a direct challenge to Trump

The rockets pose a direct challenge to Trump, who issued a threat to Iran on Tuesday, just hours before the attacks began. "If Iran does anything that it shouldn't be doing, they will be suffering the consequences and very strongly," the President said.

In the immediate aftermath of Soleimani's killing, Trump repeatedly stressed that the deadly drone strike was meant to reduce violence. "We took action last night to stop a war," he told reporters a day after the attack. "We did not take action to start a war."

Trump's message later shifted to warning of a "disproportionate" attack that could include targeting Iran's cultural sites, a war crime.

Iranian Foreign Minister Javad Zarif told CNN early Tuesday that Tehran was not intimidated.

"His threats will not frighten us," Zarif said.

"Disproportionate response is a war crime. But he doesn't care it seems about international law," Zarif said of Trump. "But has he made the US more secure? The American people are more secure? Are the Americans today welcome in this region?"

Iran's decision to attack is a "huge gamble," said Thomas Juneau, an assistant professor and Iran expert at the University of Ottawa who said that leaders in Tehran have included Trump's domestic political concerns in their calculations. The President ran on a platform of ending US involvement in the messy Middle East entanglements.

"Iran assesses Trump does not want to get bogged down in a large scale war in the Middle East, and that this gives it more margin to maneuver," Juneau wrote on Twitter. "Needless to say, this is a HUGE gamble given how unpredictable Trump is."

Randa Slim, a senior fellow at the Middle East Institute, said Iran's attack was a foregone conclusion after the Ayatollah announced there would be retaliation. Further escalation will depend on Trump's ability to absorb the attack and a few casualties, she said.

She noted the pattern often seen in attacks on Israel and Hezbollah, in which one side attacks, the other responds, the attacks are absorbed and mediation begins through a third party.

"From Trump's rhetoric, he doesn't seem as if he's willing to absorb any Iranian retaliation -- no matter how proportional it is to the US attack that killed Soleimani -- and that means we're locked into an escalatory spiral that will push us into war that will unfold on Iranian territory, but also in the rest of region, including Iraq," Slim said.

Lawmakers urged calm. Rep. Eliot Engel, the New York Democrat who heads the House Foreign Affairs Committee, told CNN's Erin Burnett there's a need to "tone down the rhetoric on all sides and see how we can extricate ourselves from this nightmare because I don't think the American people want to go to war."

House Speaker Nancy Pelosi was discussing the situation in Iran when she was handed a note with news of the attack, lawmakers who attended the meeting said.

Rep. Dan Kildee, a Michigan Democrat, said she paused the discussion to tell the members of the Steering Committee of the news.

"Pray," Pelosi told members, according to Rep. Debbie Dingell.