US Defense Secretary Ashton Carter warned on Sunday that the fight to wrest control of Raqqa, the Islamic State group stronghold in Syria, "will not be easy."

"The effort to isolate, and ultimately liberate, Raqqa 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 group.