No future for Assad and his family in Syria, says US top diplomat Tillerson

US Secretary of State Rex Tillerson said on Thursday that Washington sees no future for President Bashar al-Assad in Syria's government, insisting "the reign of the Assad family is coming to an end."

Tillerson made the comments after what he called a "fruitful" discussion with UN Syria envoy Staffan de Mistura, who is expected to reconvene UN-mediated peace Syrian talks in the coming weeks.

"The United States wants a whole and unified Syria with no role for Bashar al-Assad in the government," Tillerson told reporters.

"The reign of the Assad family is coming to an end, and the only issue is how that should be brought about."

"We do not believe there is a future for the Assad regime, the Assad family," he added.

Seven rounds of talks have achieved only incremental progress toward a political deal, with negotiations deadlocked over the fate of Assad.

The opposition is demanding that any settlement provide for a transition of power to end Assad's rule but as government forces make gains on the battleground, there is little likelihood of a breakthrough on that issue.

The US' top diplomat said that an exit of Assad should be done through the Geneva process led by de Mistura, but that such a departure was not a "prerequisite" for that process to start.

The comments came before de Mistura addressed the UN Security Council by video-conference on Thursday.

He announced plans to reconvene the intra-Syrian talks on Nov 28. It will be the eighth round of talks under his guidance since early 2016.

Opposition delegations, which do not include the Islamic State group or other UN-designated terrorist groups, have never spoken face-to-face with Syrian government envoys under de Mistura's mediation.

Tillerson also sought to play down any idea that the Syrian government's advances might amount to a "triumph" for Iran, which has been a key backer of Assad.

"I see Iran as a hanger-on," Tillerson said.

"Iran has not been successful; the Russian government has been more successful. We have had success. I don't think that Iran should be given credit for the defeat of ISIS in Syria.

The Russian-backed Syrian government forces as well as US-backed, Kurdish-led troops have recently significantly reduced territory controlled by IS across Syria.