Syrian government forces backed by Russian airpower and allied militias opened a new front against rebel fighters south of second city Aleppo on Friday.

The fresh offensive came as Turkey said it had downed an "air vehicle" of unknown origin that violated its air space close to the Syrian border.

And a monitoring group said that more than 250,000 people have now been killed since the conflict began with anti-government protests in March 2011.

The Aleppo offensive is the fourth that President Bashar al-Assad's regime has launched since Moscow began an air campaign on September 30.

Aleppo city has been divided between regime control in the west and rebel control in the east since mid-2012.

The situation is largely reversed in the countryside surrounding the city, which was once the country's economic hub, and the front lines have been static for some time.

"A major military operation began on Friday in the southern parts of Aleppo province with the participation of our allies and our friends," a security source on the ground told AFP.

He clarified that "allies" referred to Russian support, and "friends" to Iranian and Hezbollah fighters.

The joint command for the Aleppo operation issued a statement saying the army had begun operations on the western and southern outskirts of Aleppo "to liberate you from the armed terrorist groups."

It warned that any resident or village cooperating with rebels would be a military target, but that anyone raising a white flag would be safe.

- 'Dozens' of Russian strikes -

"The young men who cooperate with our forces during the operation will have their situation taken care of," it added.

The Britain-based Syrian Observatory for Human Rights said the regime had taken control of two villages during the offensive 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 regime forces began an operation north of the city of Homs, which lies in the centre of the country and is largely under government control.

Syrian state media said the operation was intended to restore "safety and security" to the area and would not target civilians.

- Turkey downs 'air vehicle' -

But the Observatory said that most of the 60 people killed in the fighting on Thursday were civilians.

"Eight children, 22 women, and 13 civilian men were killed in the fighting, shelling and strikes," Abdel Rahman said.

The violence also killed 17 rebel fighters, he added.

Since Syrian forces began ground operations in tandem with Russian air strikes on October 7, their focus has appeared to be a stretch of highway between Aleppo and Homs.

The fighting north of Homs on Thursday centred around the town of Talbisseh on the highway, and regime operations in neighbouring Hama province have also focused on the key road.

Syrian forces have also sought to reinforce the coastal province of Latakia, a regime stronghold, fighting rebels in the north of the province.

Moscow says its strikes have targeted the Islamic State group and other "terrorists."

It said on Friday that it had hit more than 380 IS targets in Syria since the start of its bombing campaign.

But rebels and their backers accuse Russia of targeting moderate and Islamist opposition forces rather than IS.

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

On Friday, Turkey said it had shot down an unidentified aircraft, but it was not immediately clear if it was manned or a drone.

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

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

The Syrian conflict began with anti-government protests in March 2011 but has since spiralled into a multi-front civil war involving the regime and its allies, the rival jihadists of IS and Al-Qaeda, and Kurdish militia as well as a string of rebel groups.

The Observatory said on Friday that the death toll was now more than a quarter of a million, over 74,000 of them civilians.

Source: AFP