US personnel checked for brain injuries after ballistic missile attack on Al-Asad Air Base in Iraq

U.S. personnel were injured in a ballistic missile attack on Al-Asad Air Base in Iraq on Saturday, two U.S. officials said.

The attack was one of the largest-scale ballistic missile attacks to date that the U.S. has seen carried out by Iran-backed militias in Iraq, deputy Pentagon press secretary Sabrina Singh told reporters on Monday. Two U.S. service members were diagnosed with traumatic brain injuries and have since returned to duty, she said.

U.S. Central Command confirmed the attack Saturday evening.

Multiple ballistic missiles and rockets targeted the base and, while most were intercepted by the base's air defenses, some made impact, the CENTCOM statement said.

The use of more powerful ballistic missiles, far rarer than rockets or one-way attack drones, comes at a time of growing tension in the region as the Israel-Hamas war passes 100 days.

U.S. and coalition forces have faced increased threats from Iranian-backed Shia militias since the start of the war. As of Thursday, U.S. and coalition forces had come under attack more than 143 times in Iraq and Syria since October 7; Saturday's incident appears to be the second time ballistic missiles have been used to attack the U.S..

The Islamic Resistance in Iraq, an Iranian-backed militia group, claimed responsibility for Saturday's attack.

In a statement, the group emphasized its commitment to resisting American "occupation forces" in the region and cited the attack as a response to what it referred to as the "Zionist entity's massacres" against the Palestinian people in Gaza. The group did not provide any evidence for its claim.

In November, militants in Iraq fired close-range ballistic missiles at coalition forces, prompting the U.S. to carry out airstrikes against facilities used by the Iranian-backed group Kataib Hezbollah, which has been responsible for many of the rocket and drone attacks against the coalition.

U.S. forces in Iraq and Syria operate as part of the coalition to defeat ISIS. But the Iraqi government has called for international forces to ultimately leave the country after U.S. strikes targeting Kataib Hezbollah facilities - which marked the first time the Biden administration had carried out strikes in Iraq since the war in Gaza began - caused tension with the Iraqi government.

"We believe that reorganizing the relationship through ending its presence will prevent more tensions and entanglement of internal and regional security issues," Iraqi Prime Minister Mohammed Shia al-Sudani told Reuters earlier this month.

The UN secretary-general's special representative for Iraq has also warned that the region is at a "critical juncture" with the war in Gaza, saying in a statement that Iraq is at risk of being further drawn into the conflict.

"Despite the Government's efforts to prevent the escalation of tensions, continued attacks - originating from within and outside of Iraq's borders - stand to undo the hard-won stability of the country and the achievements it has made in recent years," the statement said.

The Pentagon has maintained that the presence of its forces is still at the invitation of the Iraqi government and nothing has changed. National security adviser Jake Sullivan met with al-Sudani earlier this week in Davos, Switzerland, and the two discussed the "importance of sustaining a strong bilateral partnership," the White House said in a readout.

The U.S., separately, is also facing threats in the Red Sea and the Gulf of Aden. Iranian-backed Houthi rebels have launched repeated attacks on international shipping lanes, including at least two strikes on U.S.-owned vessels this week, leading some of the world's largest shipping companies to avoid the critical waterway.

The U.S. has carried out a series of increasingly frequent strikes targeting Houthi assets and President Joe Biden said Thursday the strikes will continue, while conceding that the U.S. strikes haven't deterred the group's attacks.

CNN's Kaanita Iyer, Natasha Bertrand, Mohammed Tawfeeq, Hamdi Alkhshali, Haley Britzky and Donald Judd contributed to this report.