Pentagon spokesman John Kirby made no mention of possible civilian casualties, but said than nine buildings were destroyed in the pinpoint strike on the outpost near Albu Kamal.

The US military said was used by Iran-supported Iraqi armed groups.

"We believe right now there was likely one militia member killed and two militia members wounded," Kirby said.

"We'll continue to assess ... and if that changes we will certainly let you know," he added.

Shortly after the early Friday attack, the Syrian Observatory for Human Rights said 22 fighters from Iraq's state-sponsored Hashed al-Shaabi paramilitary force were killed.

The strike was in retaliation for three recent missile attacks on installations used by US and coalition forces in Iraq.

Those were believed launched by Iran-backed militias that operate under the umbrella of Hashed al-Shaabi.

It was the first military strike by the administration of Joe Biden in the region and came as Biden seeks to resume negotiations with Tehran over limiting its nuclear program.

Biden was "sending an unambiguous message that he's going to act to protect Americans," White House spokeswoman Jen Psaki said on Friday.

Secretary of Defense Lloyd Austin said they were confident the target was being used by "the same Shia militia that conducted the strikes" in Iraq against US-used facilities.

The administration has stressed that the action was meant as a warning and to avoid further escalation of tensions between Tehran and Washington.

"This was really designed to do two things: to remove that compound from their utilization of it as an entry control point from Syria into Iraq," Kirby said.

"And, two, to send a very strong signal that we're not going to tolerate attacks on our people and our Iraqi partners."