As many as 2,000 Iranian and Iran-backed forces are currently in Syria helping regime troops in an offensive near Aleppo that is being coordinated with Russia, a U.S. official said Friday.
The fighters were helping Syrian forces loyal to President Bashar Assad, backed by Russian air power, as they opened a new front against rebel fighters southeast of Aleppo.
"We are now seeing a coordinated effort between Iran and Russia to assist Assad with fighting opposition groups," the official said, speaking on condition of anonymity.
"There could be as many as 2,000 Iranian-enabled forces," he added.
The official defined those forces as consisting of Iranians including Quds special forces, Iranian-funded proxies and Lebanese Hizbullah, noting it was difficult to determine a precise number of fighters.
Military cooperation between Iran and Russia in Syria is rooted in a visit General Qasem Soleimani, who heads the Quds forces, made to Moscow in August, the official said.
Russians at the time denied Soleimani had visited Moscow.
Colonel Pat Ryder, the spokesman for the U.S. military's Central Command, said there were clear "indications" Iranian forces were in Syria.