US-backed fighters yesterday launched a fierce assault against a dwindling pocket of territory held by the Islamic State group in east Syria, a commander and a war monitor told AFP.

The Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, have been closing in for months on the town of Hajin in eastern Deir Ezzor province.

Yesterday, they began an offensive for the IS-held town itself. An SDF commander said the assault, relying heavily on artillery and US-led coalition air strikes, had killed at least 15 IS fighters. The Syrian Observatory for Human Rights monitor said the IS death toll was at least 17.

"Our forces today began attacking the last bastions of Daesh in Hajin, with intense artillery and air support," said the SDF commander, who spoke on condition of anonymity and used the Arabic acronym for IS.

"The clashes will be fierce in Hajin because Daesh has reinforced their positions, but we will take control of it," the commander told AFP.

"The operation to end Daesh's presence in this pocket began today, with the heaviest air strikes, artillery fire, and ground attacks in months by the SDF and the coalition," said Observatory head Rami Abdel Rahman.

Meanwhile, UN's new humanitarian chief warned yesterday that a large-scale military operation against the rebel-held Syrian province of Idlib could create "the worst humanitarian catastrophe" of this century.

"There needs to be ways of dealing with this problem that don't turn the next few months in Idlib into the worst humanitarian catastrophe with the biggest loss of life in the 21st century," Mark Lowcock told reporters in Geneva.