"Three U.S. service members were killed -- and many wounded -- during an unmanned aerial drone attack on our forces stationed in northeast Jordan near the Syria border," Mr. Biden said in a statement on Sunday. "While we are still gathering the facts of this attack, we know it was carried out by radical Iran-backed militant groups operating in Syria and Iraq." Speaking later in Columbia, S.C., Mr. Biden said, "We lost three brave souls." The president then led a moment of silence, before adding, "We shall respond." Defense Secretary Lloyd J. Austin III also held Iran-backed militias responsible for the continued attacks against U.S. troops in the region, but he did not identify which country the attack was launched from.