IS defeat on the ground is 'critical milestone': US envoy

An American envoy to the US-led coalition fighting the Islamic State group hailed the demise of the extremist group's self-declared "caliphate" in eastern Syria on Saturday as a "critical milestone".

"This critical milestone in the fight against ISIS delivers a crushing strategic blow and underscores the unwavering commitment of our local partners and the global coalition to defeat ISIS," William Roebuck said, using an alternative acronym for IS.

But the coalition envoy warned that the fight against the jihadists was not yet over.

"While we have completed the territorial defeat of ISIS in Syria and Iraq, we still have much work to do to achieve an enduring defeat of ISIS," he said at a main staging base for the US-backed Syrian Democratic Forces whose Kurdish-led fighters defeated IS.

"The campaign is not over. ISIS... remains a significant threat," Roebuck said at the Omar oil field in eastern Syria.

IS's loss on Saturday of its last redoubt in the village of Baghouz signalled the demise of the "caliphate" that the extremist group declared over a large swathe of Syria and neighbouring Iraq in 2014.

But the jihadists still retain a presence in eastern Syria's vast desert and have claimed deadly attacks in areas controlled by US-backed forces.

"We will continue to support the coalition's operations in Syria to ensure... this enduring defeat," Roebuck said.

President Donald Trump and US Secretary of State Mike Pompeo "have been forceful in saying that we will do what is necessary in the region, including here in Syria, and across the globe to ensure the defeat of this threat.