US President Donald Trump on Saturday said the end of the Islamic State group's "caliphate is in sight" following the fall of the former IS stronghold Raqa, and a transition can soon begin to set conditions for lasting peace in Syria.

"With the liberation of ISIS's capital and the vast majority of its territory, the end of the ISIS caliphate is in sight," he said in a statement, using an alternate acronym for IS.

"We will soon transition into a new phase in which we will support local security forces, de-escalate violence across Syria, and advance the conditions for lasting peace, so that the terrorists cannot return to threaten our collective security again," he said.