Beirut: Syrian troops backed by Russian airpower and allied militias opened a new front today against rebels around second city Aleppo, where Washington claimed up to 2,000 Iran-backed forces were deployed.
The offensive came as Turkey said it had downed a drone of unknown origin that violated its air space close to the Syrian border, and a monitor said the toll in the conflict had risen to more than 250,000 people.
The Aleppo offensive is the fourth that President Bashar al-Assad's regime has launched since Moscow began an air campaign on September 30.
Control of Aleppo city, once the country's economic hub, has been divided between the regime and rebels since mid-2012. There and in the surrounding countryside the front lines have long been static.
A security source said the operation in southern Aleppo province was backed by Russian strikes and fighters from Iran and Lebanon's Hezbollah militia.
And a US official said as many as 2,000 Iranian and Iran-backed forces were aiding the offensive.
"We are now seeing a coordinated effort between Iran and Russia to assist Assad with fighting opposition groups," the official said.
The joint command for the Aleppo operation said the army had begun operations on the western and southern outskirts of Aleppo "to liberate you from the armed terrorist groups."
The Britain-based Syrian Observatory for Human Rights said the regime had taken control of two villages so far, and that Russian warplanes were pounding two others.
Observatory head Rami Abdel Rahman said Russia had carried out dozens of strikes on the area, which is controlled by a patchwork of rebel groups, including moderates, Islamists and Al-Qaeda affiliate Al-Nusra Front.
The Aleppo fighting comes a day after government forces began an operation north of Homs city, which lies in the centre of the country and is largely under government control.