A new round of UN-led talks aimed at ending Syria's six-year war will take place in Geneva from November 28, UN envoy Staffan de Mistura said Thursday.

The talks will follow up on a meeting next week in the Kazakh capital of Astana between Russia, Iran and Turkey that is also aimed at achieving a settlement.

De Mistura told the UN Security Council that with the defeat of the Islamic State in their strongholds of Raqa and Deir Ezzor, the Syrian peace process had reached a "moment of truth."

"We need to get the parties into real negotiations," the envoy said.

Seven rounds of talks have achieved only incremental progress toward a political deal, with negotiations deadlocked over the fate of Syrian President Bashar al-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 Geneva talks -- the eight round convened by the United Nations -- must focus on steps toward drafting a new constitution and holding UN-supervised elections in Syria, De Mistura said.

More than 330,000 people have died and millions have been driven from their homes in the conflict in Syria.