The U.S. struck facilities tied to Iran-backed militias after an attack earlier on Monday injured three service members in the most serious attack on American troops in Iraq and Syria, the Pentagon and White House confirmed.

U.S. military forces conducted “necessary and proportionate” strikes on the facilities in direct response to the Monday attack by Iran-affiliated Kataib Hezbollah group at Erbil Air Base in Iraq, Secretary of Defense Lloyd Austin said in an emailed statement to the Daily Caller News Foundation. Despite more than 100 claimed attacks on U.S. and coalition bases in Iraq and Syria since Oct. 17, according to The Associated Press, Monday’s was the first to result in critical injuries to U.S. troops and could signal an further escalation in tensions in the region.

“Today, at President Biden’s direction, U.S. military forces conducted necessary and proportionate strikes on three facilities used by Kataib Hezbollah and affiliated groups in Iraq,” Austin said.

President Joe Biden was immediately briefed on the suicide drone attack early Monday and requested response options from the Department of Defense against the responsible groups, the National Security Council said in a statement. The president received options in a call Monday afternoon with the defense secretary and members of the national security team.

“During that call, the President directed strikes against three locations utilized by Kataib Hezbollah and affiliated groups focused specifically on unmanned aerial drone activities,” the statement said.

“These precision strikes are a response to a series of attacks against U.S. personnel in Iraq and Syria by Iranian-sponsored militias, including an attack by Iran-affiliated Kataib Hezbollah and affiliated groups on Erbil Air Base earlier today, and intended to disrupt and degrade capabilities of the Iran-aligned militia groups directly responsible,” Austin said.

Neither statement offered more specific details on the facilities or a damage assessment.

Monday’s attack injured three U.S. personnel and rendered one service member critically injured, according to the statement. Previous attacks have hurt dozens of servicemembers, but the Pentagon has classified such wounds as minor.

U.S. fighter jets bombed facilities belonging to Iran’s Islamic Revolutionary Guard Corps (IRGC) and its regional proxy groups three times since the attacks began, with Monday’s strikes marking the fourth premeditated retaliation.

U.S. CENTCOM conducts strikes against Kataib Hezbollah terrorist group targets in Iraq

In response to multiple attacks against coalition forces in Iraq and Syria, U.S. military forces conducted airstrikes against multiple facilities used by Kataib Hezbollah and affiliated groups… pic.twitter.com/mmL4WqFXq9

— U.S. Central Command (@CENTCOM) December 26, 2023

“And let me be clear – the President and I will not hesitate to take necessary action to defend the United States, our troops, and our interests. There is no higher priority. While we do not seek to escalate conflict in the region, we are committed and fully prepared to take further necessary measures to protect our people and our facilities,” Austin said.

An coalition Iran-backed militant organization going by the umbrella name Islamic Resistance of Iraq threatened to target U.S. troops in response to American military support for Israel as the war in Gaza rages.