Mr. al-Shara was once imprisoned by U.S. forces in Iraq, and then went on to found the Syrian branch of Al Qaeda. He had a $10 million U.S. bounty on his head until December of last year, shortly after he led a rebel offensive that ousted Syria’s longtime dictator, Bashar al-Assad.

The Syrian leader has been discreetly cooperating with the U.S.-led coalition against ISIS and Al Qaeda since he took control of a slice of rebel-held territory in northwestern Syria in 2016, according to Syrian officials and Western diplomats who spoke on condition of anonymity in keeping with diplomatic protocol.