Turkey and Syrian-Kurdish forces face off in Raqqa battle

The battle to retake Raqqa - Islamic State's de facto capital in Syria - "will not be easy" the US defence secretary has said, as Washington puts its confidence in Kurdish-dominated Syrian Democratic Forces (SDF).

The SDF on Sunday announced the start of operations to deny IS its so-called caliphate by liberating the northern Syrian town, while Iraqi forces simultaneously continue their advance on Mosul.

"As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of IS' caliphate and disrupt the group's ability to carry out terror attacks against the United States, our allies and our partners," Ash Carter said.

The operation, referred to as "Wrath of the Euphrates", will involve 30,000 Kurdish and Arab soldiers.

The US sees the SDF as representing the most effective rebel fighting force in Syria, but it is dominated by the Kurdish People's Protection Units, or YPG.

That has complicated the operation, with Turkey wanting a role but viewing the YPG as synoymous with the PKK, which Ankara designates as a terrorist organisation.

SDF spokesman Talal Sello claimed on Sunday it had brokered a deal with Washington "that there will be no role for Turkey or the armed factions allied with it in the operation" to capture Raqqa.

Fighting would involve two stages, he added, "first liberating the countryside around Raqqa and isolating the city, and second taking control of the city".

However later on Sunday, US chairman of the joint chiefs of staff Joe Dunford, met with his Turkish counterpart Hulusi Akar, and confirmed the NATO allies "will work together on the long-term plan for seizing, holding and governing Raqqa".

He added that Raqqa's capture could not go ahead "without incorporating the Turks and their perspective into our plans".

"Obviously as a close ally, we really just want to make sure that we're completely tight as we work through some challenging issues," he said.

He said as the operation progresses, the US would negotiate with Turkey and other groups to determine who would seize and govern Raqqa.

"We always knew the SDF wasn't the solution for holding and governing Raqqa," Dunford said. "What we are working on right now is to find the right mix of forces for the operation."

"[The operation needs] a predominantly Arab and Sunni Arab force," he said. "And there are forces like that. There is the moderate Syrian opposition, the vetted Syrian forces and the Free Syrian Army forces, and there is some initial outreach to forces in Raqqa proper."

He added it could take anti-IS forces "months" to isolate Raqqa, before an operation to capture it can begin.

Agencies contributed to this report.