PALMYRA, Syria — At least four U.S. service members were injured, including some critically, when an unidentified attacker opened fire on their joint patrol with Syrian forces in central Syria’s Palmyra on Saturday, a U.S. official told Reuters.

Syrian state news agency SANA cited a security source saying that two Syrian service members were also wounded and that the assailant was killed. The source did not provide further details.

The U.S. official, speaking on condition of anonymity, noted that the number of wounded and the severity of their injuries could change as more information becomes available. U.S. helicopters evacuated the injured to a U.S. base in the Al-Tanf region near the Iraqi border.

Earlier, local Syrian officials told Reuters that a convoy of Syrian military and American-led coalition forces fighting Islamic State came under fire while on patrol. The U.S. maintains troops in northeastern Syria as part of a decade-long mission supporting Kurdish-led forces against Islamic State.