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A deadly drone attack on a small U.S. military outpost in Jordan has killed three American soldiers and injured over 30 service members. The attack, which U.S. officials say was likely carried out by Iran-backed militants operating in Syria or Iraq, marks the first time U.S. forces have been killed by enemy fire in the Middle East since fighting erupted in Gaza.

Speaking in South Carolina, President Biden vowed retaliation, saying “We shall respond.” He called the attack at Tower 22 near the Syrian border a “significant escalation” of an already tense situation. While the specifics are still being investigated, Biden said the U.S. knows the strike was perpetrated by “radical Iran-backed militant groups” and promised to “hold all those responsible to account at a time and in a manner of our choosing.”

Iran denied involvement in the attack, but a statement by the Islamic Resistance in Iraq, an umbrella group of Iran-backed militias, claimed responsibility for strikes along the Jordan-Syria border near the U.S. outpost. Eight injured service members have been evacuated for further care, and the number of casualties is expected to rise as troops are evaluated for traumatic brain injuries.

Biden honored the fallen service members, asking for a moment of silence at a South Carolina church. He said they “embodied the very best of our nation” in their selfless commitment to duty. It remains unclear why air defenses failed to stop the drone, the first known attack on Tower 22. About 350 U.S. Army and Air Force personnel are stationed there to support the coalition effort to defeat ISIS. The strike comes after over 150 attacks on U.S. forces in Iraq and Syria this year. The U.S. has retaliated against Iran-backed groups on several occasions, most recently last week.