British Prime Minister Rishi Sunak said he was concerned about tensions in the Middle East and urged Iran to de-escalate, after an attack that killed three US service members on the Syrian-Jordanian border.

Sunak said Britain "absolutely condemned" the drone attacks, which US President Joe Biden said had been carried out by Iran-backed militants.

"We are concerned and would urge Iran to continue to de-escalate tensions in the region," Sunak told broadcasters on Monday.

"We stand resolutely with our allies to bring stability and peace to the region and that's what we continue to work towards."

The attack, which also wounded 34 US service personnel, is the first deadly strike against US forces since the Israel-Hamas war erupted in October, and marks a major escalation in tensions that have engulfed the Middle East.

Iran's mission to the United Nations said in a statement on Monday that Tehran was not involved in the attack.