The U.N.'s top envoy for Syria says new talks between Syria's government and opposition will start Nov. 28 and is urging those with influence to press both sides to be ready "to negotiate substance."

Staffan de Mistura told the Security Council on Thursday that he hopes a meeting in Kazakhstan's capital Astana on Oct. 30-31 co-sponsored by Turkey, Iran and Russia will "prevent further unraveling of interim de-escalation and cease-fire arrangements" in Syria.

He said by videoconference from Geneva that it's time to move on the political track to end the Syrian conflict and "now is the moment of truth."

De Mistura expressed hope that Syria's disparate opposition groups will meet in Saudi Arabia's capital Riyadh ahead of the eighth round of U.N.-led talks in Geneva starting Nov. 28.