A US-backed operation by Syrian forces to capture Raqqa, ISIL's self-declared capital in Syria, will start in the next "few days", the spokesman for the Syrian Kurdish YPG militia said on Saturday.

The Syrian Democratic Forces (SDF), an alliance of fighters led by the YPG and backed by the US-led coalition, has been encircling Raqqa since November in a multi-phased campaign to drive the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) from the city.

The assault on Raqqa will pile even more pressure on ISIL, with the hardline group facing defeat in the Iraqi city of Mosul and being forced into retreat across much of Syria, where the province of Deir Az Zor is its last major foothold. 

"The forces reached the outskirts of the city, and the major operation will start ... in the coming few days," YPG spokesman Nouri Mahmoud told Reuters news agency by phone.

He was confirming a report citing the spokeswoman for the Raqqa campaign, Jihan Sheikh Ahmed, as indicating a new phase to storm Raqqa would start in the "coming few days".

The remarks made in an interview with a local media outlet were circulated by an SDF-run Whatsapp group.

A spokesman for the US-led coalition against ISIL said it would not comment on the timeline for the next phase of operations to retake the Syrian city, located on the River Euphrates some 90 km (56 miles) from the Turkish border.

The spokesman, Colonel Ryan Dillon, said the SDF were "advancing closer and closer every day", having moved to within 3km of Raqqa to the north and east.

READ MORE: US begins sending weapons to Kurdish YPG in Syria

To the west, the SDF were less than 10km away, he said in an email interview.

The US said on Tuesday it had started distributing weapons to the YPG to help take Raqqa, part of a plan that has angered NATO-ally Turkey, which is worried by growing Kurdish influence in northern Syria.

Turkey views the YPG as the Syrian extension of the outlawed Kurdistan Workers Party, or PKK, which has launched attacks in southeast Turkey since 1984 and is considered a "terrorist" group by the United States, Turkey and Europe.

The US-led coalition has said some 3,000 to 4,000 ISIL fighters are thought to be holed up in Raqqa city, where they have erected defences against the anticipated assault.

 

The US-led coalition has provided air support and special forces to help the SDF operations near Raqqa.

"The battle will not be easy," Mahmoud said. "Of course, [ISIL] has tunnels, mines, car bombs, suicide bombers, and at the same time it is using civilians as human shields."

Once Raqqa falls, Deir Az Zor province in eastern Syria will be ISIL's last major foothold in Syria and Iraq.

"[ISIL] will resist because Raqqa is its capital and, if Raqqa goes, that means the entire caliphate is gone," Mahmoud said.

Source: News agencies