Dec 13 (Reuters) - As many as four U.S. service members were injured, including some critically, when an unidentified attacker opened fire on their patrol in the central Syrian town of Palmyra on Saturday, a U.S. official told Reuters.

Syrian state news agency SANA quoted a security source as saying that two Syrian service personnel were injured and the assailant was killed, without providing further details.

The number of wounded and the severity of their injuries could change as more information came in, said the U.S. official, who spoke on condition of anonymity.

Earlier, two local Syrian officials told Reuters that a convoy of Syrian military forces and U.S. forces from the American-led coalition fighting Islamic State came under fire.

The U.S. has troops stationed in northeastern Syria as part of a decade-long effort to help a Kurdish-led force there.

The source told SANA U.S. helicopters evacuated the injured to a U.S. base in Syria's Al-Tanf region near the Iraqi border.

(Reporting by Syria Bureau and Idrees Ali, Editing by Timothy Heritage and Alexander Smith)