Trump says end of Islamic State 'caliphate is in sight' after Raqqa's fall

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

The United States and its allies will support diplomatic negotiations "that end the violence, allow refugees to return safely home, and yield a political transition that honors the will of the Syrian people," he said in a statement.

The declaration came four days after US-backed Kurdish-led forces recaptured Raqqa, the capital of IS's self-proclaimed caliphate and its last major stronghold in Syria.

Trump said the entire city has been liberated from IS control, which he said represented a "critical breakthrough" in the global struggle against the militant group.

"With the liberation of ISIS's capital and the vast majority of its territory, the end of the ISIS caliphate is in sight," Trump said, using an alternative 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.