AIN ISSA, Syria, Nov 6 (Reuters) - U.S.-backed rebels said on Sunday they were launching an operation to retake the Syrian city of Raqqa, the de facto capital of Islamic State.

The attack ratchets up pressure on the militant group at a critical moment, with its fighters already battling an offensive by Iraqi security forces on their remaining Iraqi stronghold in the northern city of Mosul.

The U.S.-backed Syria Democratic Forces (SDF), an alliance of Kurdish and Arab armed groups, first announced on Sunday that a campaign to retake Raqqa would begin within hours, with U.S. forces providing air cover. Soon afterwards, it said that the operation, called Euphrates Anger, had begun.

“The general command of the Syria Democratic Forces announces the blessed start of its major military campaign to liberate the city of Raqqa,” Jehan Sheikh Amad, an SDF spokeswoman, told a news conference in the Syrian town of Ain Issa.

The SDF called on Raqqa’s civilians to avoid areas where Islamic State militants are present and to go to “liberated territory.”

An attack on Raqqa has been long expected, with U.S. Defence Secretary Ash Carter saying on Oct. 25 that the battle to retake it would “overlap” with the assault on Mosul.

The top U.S. military commander in Iraq, Army Lieutenant General Stephen Townsend, said last month that the U.S.-led coalition fighting Islamic State wanted to move urgently to isolate Raqqa because of concerns about the group using the city as a base to plan and launch attacks against targets abroad.

France has also pushed for simultaneous action on both fronts. President Francois Hollande said last month there was evidence that Islamic State fighters were fleeing to Raqqa, and that everything must be done to stop them regrouping there.

French Defence Minister Jean-Yves Le Drian said on Sunday that an offensive on Raqqa should be launched while the battle to push the group out of Mosul is under way.

“We have to go to Raqqa ... it will automatically be local forces that will liberate Raqqa even if French forces, U.S. forces, the coalition contribute with air strikes to dismantle Daesh,” Le Drian told Europe 1 radio, using an Arabic acronym for Islamic State.

“Mosul-Raqqa can’t be disassociated because Islamic State and the territories it occupies span that area,” he said.

Since it was formed in early 2015, the SDF has seized large swathes of territory along the Syria-Turkey border from Islamic State and pushed the jihadist group back to within 30 km (18 miles) of Raqqa.