US President Joe Biden pledged to hold accountable those responsible for the killing of three US military forces in a nighttime drone attack on a small US site in Jordan, and placed the blame on Iranian-backed armed groups.

At least twenty others were injured in the attack.

Biden said in a statement: America’s heart is heavy. Last night, three American service members (soldiers) were killed - and many were injured - during a drone air attack on our forces stationed in northeastern Jordan near the Syrian border.”

Biden continued: “While we are still gathering the facts of this attack, we know that it was carried out by Iranian-backed extremist armed groups operating in Syria and Iraq.”

Biden pledged that the United States “will hold all those responsible accountable at a time and in the manner we choose.”

He described the three fallen soldiers as “patriots,” praised their courage and expressed his sadness, calling it “a despicable and completely unjust attack.”

"Together, we will uphold the sacred commitment we have to their families. We will strive to be worthy of their honor and courage. We will continue their commitment to fighting terrorism," the president said.

Biden was briefed on the attack on Sunday morning in South Carolina, where he is campaigning for re-election, according to the White House. Secretary of Defense Lloyd Austin, National Security Advisor Jake Sullivan, and Deputy National Security Advisor John Feiner provided the briefing.

The number of service members injured in the attack on a US base in Jordan now stands at more than 30, and is likely to rise further as they seek treatment for symptoms consistent with traumatic brain injury, two US officials said.

Officials said the attack drone that struck during the night landed near residential neighborhoods on the base.

The base that was attacked is a small American site called Tower 22, near the border with Syria. It is unclear why the air defenses failed to intercept the drone, which appears to be the first known attack on Tower 22 since the attacks on American and coalition forces in the region began last October 17.

US forces are at the site as part of an "advise and assist" mission with Jordan.