'Fight will not be easy' in Islamic State-held Raqa: US defense chief

US Defense Secretary Ashton Carter warned on Sunday that the fight to wrest control of Raqa, the Islamic State 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 defense chief added.

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

The start of the assault by the Syrian Democratic Forces (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 IS control after the jihadists suffered a string of territorial losses in Iraq and Syria over the past year.

The US-led coalition battling IS 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 on Sunday.