Fight will not be easy' in ISIS-held Raqa: US Defence Chief Ashton Carter

Washington: US Defence Secretary Ashton Carter warned on Sunday that the fight to wrest control of Raqa, the ISIS group stronghold in Syria, "will not be easy."

"The effort to isolate, and ultimately liberate, Raqa marks the next step in our coalition campaign plan," Carter said in a statement.

"As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of ISIL's caliphate and disrupt the group's ability to carry out terror attacks against the United States, our allies and our partners," he said, using an alternative name for the jihadist group.

"The international coalition will continue to do what we can to enable local forces in both Iraq and Syria to deliver ISIL the lasting defeat it deserves," the US defence chief added.

Carter issued his remarks as US-backed Kurdish-Arab forces launched an offensive on the ISIS group's de facto Syrian capital Raqa, increasing pressure on the jihadists who are already battling Iraqi troops in Mosul.

The offensive's "first phase will be to isolate Raqa," the US Central Command told AFP, adding that the Syrian Democratic Forces (SDF) were vital to the mission.

"We believe the inclusion of fighters from the local population is an important advantage to the SDF."

A US official source told AFP that recruitment of local forces is ongoing, adding that "it will likely be Arab forces forces that largely mirror the population of the city that will have to go into the city."

The start of the assault by the SDF came as Iraqi forces fought inside Mosul for the third day running, with the jihadists putting up fierce resistance.

The two cities are the last major urban centers under ISIS control after the jihadists suffered a string of territorial losses in Iraq and Syria over the past year.

The US-led coalition battling ISIS is backing both assaults, hoping to deal a knockout blow to the self-styled "caliphate" the group declared in mid-2014.

The operation, dubbed "Wrath of the Euphrates", involves some 30,000 fighters and began late Saturday.