BEIRUT: The next phase of a U.S.-backed campaign against ISIS in Syria aims to completely encircle its stronghold of Raqqa, severing the city from areas held by the extremist group in Deir al-Zor to the south, a Kurdish military source said on Tuesday.

The offensive by the Syrian Democratic Forces (SDF) alliance, including the powerful Kurdish YPG militia, got underway in November with the aim of gradually encircling Raqqa city, Islamic State's de facto capital in Syria.

Cutting off Raqqa city from ISIS strongholds in Deir al-Zor province to the southeast would be a major blow against the group in Syria. The campaign is backed by the U.S.-led coalition against ISIS.

"The coming phase of the campaign aims to isolate Raqqa completely," said the Kurdish military source, who declined to be named. "In order to accomplish this requires reaching the Raqqa-Deir al-Zor road," the source said. "This mission will be difficult."

Separately, SDF spokesman Talal Silo said preparations were underway for "new action" against ISIS starting in "a few days", but declined to give further details.