Smoke rises from buildings following an air strike by Syrian government forces in the eastern city of Deir Ezzor, on October 31, 2017, during an operation against Islamic State (IS) group jihadists. Photo: AFP

Syria's army and allied fighters have retaken the city of Deir Ezzor from the Islamic State group in a Russian-backed operation, a monitor said on Thursday.

There was no immediate confirmation from official sources, though state media earlier reported army advances in the city in the country's east.

The recapture would mean IS has lost the last of the major cities it once held, after a string of defeats elsewhere in Syria and neighboring Iraq.

The jihadist group was last month driven from its one-time de facto Syrian capital Raqa, a highly symbolic blow that illustrated the way its self-proclaimed "caliphate" is crumbling.

"Regime forces and allied fighters... with Russian air support have full control of Deir Ezzor city," said the Syrian Observatory for Human Rights, a Britain-based war monitor which relies on a network of sources inside Syria for its information.

"Fighting has ended, now there are sweeping operations under way," said Observatory director Rami Abdel Rahman.

There was no confirmation on Syria's state media, which had earlier reported that the army had recaptured three neighborhoods from IS and "tightened the siege" on the jihadists remaining in the city.

State news agency SANA said IS fighters were using loudspeakers to urge remaining members of the group "not to run away from the fighting, and to kill any member who tried to escape or surrender".

Hours earlier, a military source had said the government only held around 80 percent of the city.

"Daesh is scrambling to defend its last positions, which the army is continuing to advance on until they are eliminated completely," the source said, using the Arabic acronym for IS.

He added that the jihadists were using vehicle bombs and snipers, and alleged they were "sending female suicide bombers" to target army positions.

But the Observatory said government troops and allied fighters had advanced swiftly in a final push backed by heavy Russian air strikes.

More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests, before spiraling into a complex, multi-front war that drew in international forces and jihadists.