Biden promised retaliation after a drone attack killed 3 US troops in Jordan on Saturday.
Biden called the attack "despicable."
The attack marks the first line-of-fire deaths for US troops since the start of the war in Gaza.
A drone attack killed three US service members and injured at least 25 others in Jordan, near the Syria border, on Saturday, according to President Joe Biden.
The deaths are the first US military fatalities connected to the conflict in Gaza, which began last October.
While US officials have blamed the attack on an "Iranian-backed" group, it has not yet specified exactly which one they think is responsible, according to The Associated Press. Hamas is backed by Iran, as are Hezbollah in Lebanon and the Houthis in Yemen. The Houthis have been launching drone attacks on ships in the Red Sea for months.
Biden said the United States is "still gathering the facts" regarding Saturday's "despicable and wholly unjust" attack. He pledged retaliation.
"Have no doubt — we will hold all those responsible to account at a time and in a manner of our choosing," the president said on Sunday.
The names of the service members who were killed and injured have not yet been released by the military.
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