But whether the peace talks will go forward in Geneva, and who will be at the table when they do, remains uncertain.
A Friday start for the talks appears increasingly unlikely. The main Syrian opposition group, the High Negotiations Committee, is threatening to boycott unless the regime of Syrian President Bashar al-Assad lifts sieges and stops bombarding Syrian civilians.
The head of the committee told CNN's Christiane Amanpour that they were awaiting a response from the United Nations about those points.
"We told Mr. de Mistura that these principles are above the negotiation, that they are final (and) cannot be discussed or compromised," said Riyad Hijab, head of the High Negotiations Committee.
A source with knowledge of the talks told CNN that the opposition committee's representatives would not arrive in Geneva by Friday.
The talks were originally set to begin in Geneva on Monday, but were delayed because of ongoing discussions about who should represent the opposition, de Mistura told reporters earlier this week.
Past efforts to negotiate an end to the conflict have been hindered by strong opposition to the Assad regime, which is perceived as brutal, and by the fractured nature of the opposition, which includes ISIS as well as more moderate groups supported by the United States.
The plan for new negotiations calls for proximity talks, which means the parties will be in separate rooms, with de Mistura shuttling between them.
United Nations Syria envoy Staffan de Mistura will shuttle between parties during negotiations.
The talks would mark the first time in two years that the warring sides in Syria meet in an effort to find a political way out.
De Mistura's video message Thursday said the talks would start "in the next few days." Syrians on all sides, he said, should make sure their representatives know how important it is to strike a deal.
"We need to hear your voice to everyone who is coming to this conference ... saying this conference must be an opportunity not to be missed," de Mistura said.
CNN's Nic Robertson reported from Geneva. CNN's Catherine E. Shoichet wrote the story in Atlanta. CNN's Hamdi Alkhshali, Mick Krever, Christiane Amanpour, Faith Karimi, Tim Hume, Elise Labott and Radina Gigova contributed to this report.
