Syrian troops backed by Russian air power and allied militias opened a new front Friday against rebels around the second city of Aleppo, where Washington said 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 airspace close to the Syrian border, and a monitor said the death toll in the conflict had risen to more than 250,000.

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. The front lines there and in the surrounding countryside 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.

A US official said as many as 2,000 Iranian and Iran-backed forces were aiding the offensive.

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."

'Dozens' of Russian strikes

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 moderatesAl Qaeda affiliate Al Nusra Front.

Turkey downs unidentified drone

Russia's entry into the conflict has raised concerns about military accidents in Syria's crowded air space, where a US-led coalition against Daesh is also present.

On Friday, Turkey said it had shot down an unidentified drone that had violated its airspace near the Syrian border.

Russia said all its planes in Syria were safe and its drones "working as normal".

But the US military said "all indications" were that the Turks had downed a Russian drone, adding that the Syrian army did not appear to use the same type of aircraft.

Turkey has previously shot down Syrian government helicopters and a plane, and has accused Russia of violating its airspace in recent weeks.