COLUMBIA, S.C. - President Joe Biden said Sunday that the U.S. “shall respond” after three American troops were killed and dozens more were injured in an overnight drone strike in northeast Jordan near the Syrian border. Biden blamed Iran-backed militias for the first U.S. fatalities after months of strikes by such groups against American forces across the Middle East since the start of the Israel-Hamas war.

Biden, who was traveling in South Carolina, asked for a moment of silence during an appearance at a Baptist church’s banquet hall.

“We had a tough day last night in the Middle East. We lost three brave souls in an attack on one of our bases,” he said. After the moment of silence, Biden added, “and we shall respond.”

With an increasing risk of military escalation in the region, U.S. officials were working to conclusively identify the precise group responsible for the attack, but they have assessed that one of several Iranian-backed groups was behind it.

Biden said in a written statement that the United States “will hold all those responsible to account at a time and in a manner (of) our choosing.” Defense Secretary Lloyd Austin said “we will take all necessary actions to defend the United States, our troops, and our interests.”

Iran-backed fighters in east Syria began evacuating their posts, fearing U.S. airstrikes, according to Omar Abu Layla, a Europe-based activist who heads the Deir Ezzor 24 media outlet. He told The Associated Press that the areas are the strongholds of Mayadeen and Boukamal.

U.S. Central Command said at least 34 troops were injured by the one-way attack drone, with eight flown out of Jordan for follow-up care. It described the eight as being in stable condition.

The large drone struck a logistics support base in Jordan known as Tower 22. It is along the Syrian border and is used largely by troops involved in the advise-and-assist mission for Jordanian forces.

Central Command said approximately 350 U.S. Army and Air Force personnel were deployed to the base. The three who were killed and most of the wounded were Army soldiers, according to several U.S. officials, who spoke on condition of anonymity to give details not yet made public.

The small installation, which Jordan does not publicly disclose, includes U.S. engineering, aviation, logistics and security troops. Austin said the troops were deployed there “to work for the lasting defeat of ISIS.” Three officials said the drone struck near the troops’ sleeping quarters, which they said explained the high casualty count.

The U.S. military base at al-Tanf in Syria is just 12 miles north of Tower 22. The Jordanian installation provides a critical logistical hub for U.S. forces in Syria, including those at al-Tanf, which is near where the borders of Iraq, Syria and Jordan intersect.