WASHINGTON — An American F-15E fighter jet shot down an Iranian-made armed drone that was flying toward American-backed Syrian fighters and their advisers on Tuesday, Pentagon officials said.

The episode, over southeast Syria, was a fresh indication that the air war between forces supporting President Bashar al-Assad of Syria and the American military is likely to continue, and perhaps escalate, even though the United States has sought to keep its focus on defeating the Islamic State militants operating in Syria and Iraq.

The F-15E intercepted the drone and tried to get it to change course, but it continued to fly toward the fighters, who were at a combat outpost with their advisers northeast of the town al-Tanf. The drone was shot down shortly after midnight local time.

American officials said the downed aircraft was a Shahed 129, the same type of drone that an American warplane blasted on June 8 after the drone dropped a bomb near American-supported Syrian fighters and their coalition advisers at the same location. American officials said they did not know whether Shiite militia fighters, Syrian officers or Iranian advisers were operating the drones.