Secretary of State Rex Tillerson spoke at a ministerial meeting in Kuwait of the US-led military coalition that has been fighting IS in Iraq and Syria.

The jihadist group has lost much of the territory it once controlled when it seized large swathes of Syria and neighbouring Iraq in 2014.

"ISIS is failing but the end of major combat operations does not mean we have achieved the enduring defeat of ISIS," Tillerson said, using another acronym for IS.

"ISIS remains a serious threat to the stability of the region, our homelands and other parts of the globe," he said.

Iraq declared victory over IS in December, but the extremist group still has a presence in Syria, where the US-led coalition has backed a Kurdish-Arab alliance fighting the jihadists.

Tillerson said the United States would maintain an "ISIS-focused military presence in Syria" and "continue to train local security forces".

The US-backed campaign against IS in Syria has been complicated since Turkey-supported rebels last month launched an offensive against Kurdish militia in the northwestern region of Afrin.

The Kurdish People's Protection Units (YPG) have been a key component of the US-backed alliance fighting IS, but Washington's NATO ally Turkey says they are a "terrorist" offshoot of the outlawed Kurdistan Workers' Party (PKK).

"We remain concerned over recent events in northwest Syria and call on all parties to remain focused on defeating ISIS," Tillerson said.

"We will continue to be completely transparent with Turkey about our efforts in Syria to defeat ISIS and we stand by our NATO ally in its counterterrorism efforts."