The U.N.'s Syria envoy warned peace talks due to be held in Geneva must not fail, but members of the fractious opposition said they would miss the start on Friday.
Delegates from the Saudi-backed High Negotiations Committee said late Thursday that discussions in Riyadh over whether to attend the U.N.-brokered talks would continue the next day.
But organizers of the Geneva negotiations said no postponement was expected.
U.N. envoy Staffan de Mistura, in a video message to the Syrian people, said the talks could be their country's last chance for peace.
"Now we need to hear your voice to everyone who is coming to this conference, and saying this conference must be an opportunity not to be missed," the envoy said.
He said that after several stalled peace processes, the Geneva talks "cannot fail."
A spokeswoman for the U.N. talks, Khawla Mattar, told AFP there is "no postponement from our side."