Spearheaded by the YPG, the SDF says it has taken 65 per cent of Raqqa city from Islamic State.

U.S.-backed Syrian militias have launched an operation against Islamic State in the north of Deir al-Zor province, a statement said on Saturday.

Assaults would aim to drive the jihadist militants out of areas they hold north and east of the Euphrates river, close to the Iraqi border, said the statement from the Deir al-Zor Military Council, which is fighting as part of the Syrian Democratic Forces alliance (SDF).

A senior SDF official told Reuters on Friday they would launch attacks from the south of Hasaka, which is controlled by the Kurdish YPG militia, as part of a wider offensive to drive Islamic State out of Raqqa city and territory to its southeast.

With U.S.-led air cover and special forces on the ground, the alliance of mostly Kurdish and Arab militias is fighting to seize Raqqa, upstream along the Euphrates River.

Spearheaded by the YPG, the SDF says it has taken 65 per cent of Raqqa city from Islamic State.

In the Deir al-Zor assault, the SDF would push towards the Euphrates River from the east of the province, which borders Iraq.

The Syrian Observatory for Human Rights monitoring group reported that the SDF had already made advances against IS in Deir al-Zor after fierce clashes and seized several hills and a village in the province's northwestern countryside.

The operation will likely bring the U.S.-backed militias into closer proximity with the Syrian military and allied forces, who have advanced on Deir al-Zor city.

The Syrian army reached its enclave in Deir al-Zor city this week, on the western bank of the Euphrates, breaching an Islamic State siege that had lasted three years.

With the help of Russian air power and Iran-backed militias, the advance capped months of steady progress east against Islamic State across the desert.

The eastwards march has on occasion brought the Syrian army and its allies into conflict with U.S.-backed forces.

Still, the rival campaigns have mostly stayed out of each other's way, and the U.S.-led coalition has stressed it is not seeking war with Damascus.