BEIRUT (Reuters) - A U.S.-backed operation by Syrian forces to capture Islamic State’s Syrian “capital” of Raqqa will start in the next “few days”, the spokesman for the Syrian Kurdish YPG militia said on Saturday.

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The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias backed by the U.S.-led coalition, has been encircling Raqqa since November in a multi-phased campaign to drive Islamic State from the city where it has planned attacks on the West.

The assault on Raqqa will pile more pressure on Islamic State’s self-declared “caliphate” with the group facing defeat in the Iraqi city of Mosul and being forced into retreat across much of Syria, where Deir al-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 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 U.S.-led coalition against Islamic State 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 3 km (less than two miles) of Raqqa to the north and east.

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

The United States said on Tuesday it had started distributing arms 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 fought an insurgency in southeast Turkey since 1984 and is considered a terrorist group by the United States, Turkey and Europe.

The U.S.-led coalition has said some 3,000 to 4,000 Islamic State fighters are thought to be holed up in Raqqa city, where they have erected defenses against the anticipated assault.

The U.S.-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 (IS) has tunnels, mines, car bombs, suicide bombers, and at the same time it is using civilians as human shields.”

Once Raqqa falls, Deir al-Zor province in eastern Syria will be Islamic State’s last major foothold in Syria and Iraq.

“Daesh will resist because Raqqa is its capital and if Raqqa goes that means the entire caliphate is gone,” Mahmoud said.