US says Islamic Resistance in Iraq carried out attack on base in Jordan
The "attribution that our intelligence community is comfortable with is that this was done by the umbrella group" Islamic Resistance in Iraq - a coalition of Iranian-backed militias - said National Security Council spokesman John Kirby.
In addition to the three deaths in the drone blast on Sunday at the remote base, known as Tower 22, about 40 more troops were injured, the Pentagon says.
President Joe Biden says he has already decided on the nature of the US response to the deadly bombing, but has not detailed the plans in public - or the timing.
Kirby repeated the administration's insistence that "we will respond in a time and in a manner of our choosing, on our schedule."
"Just because you haven't seen anything in the last 48 hours, it doesn't mean that you're not going to see anything," he said, adding: "The first thing you see won't be the last thing."
One of the factions in the Islamic Resistance in Iraq grouping, Kataeb Hezbollah, announced on Tuesday that it was suspending attacks on US troops.
Kirby said "you can't take what a group like Kataeb Hezbollah says at face value."