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.