As many as 2000 Iranian and Iran-backed forces are currently in Syria helping regime troops in an offensive near Aleppo that is being coordinated with Russia, a US official says.
The fighters were helping Syrian forces loyal to President Bashar al-Assad, backed by Russian air power, as they opened a new front against rebel fighters southeast of Aleppo on Friday.
"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 2000 Iranian-enabled forces," he added.
The official defined those forces as consisting of Iranians including Quds special forces, Iranian-funded proxies and Lebanese Hezbollah, 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 US military's Central Command, said there were clear "indications" Iranian forces were in Syria.
"We have seen where Iranian forces are cooperating with Syrian regime forces," Ryder said.
Regime forces backed by Russian air support are also pushing north from Hama toward Idlib, and separately northeast from Latakia toward Idlib in a pincer movement, officials said.
Ryder said the regime push meant rebel groups, as they battle regime forces, are consequently losing ground to Islamic State jihadists.
"When they do that, they strike those forces ... enabling in some cases for (IS) to retake territory that had been lost to these opposition groups, so clearly not helpful," Ryder said.