UN Special Envoy for Syria Geir Pedersen said that representatives of seven Arab and Western states, known as the small group, are due to meet in the Swiss city of Geneva on Friday.
The talks would come ahead of the first meeting of Syria's Constitutional Committee next week, Pedersen told Reuters on Thursday.
Senior officials from the so-called Astana three - Russia, Iran and Turkey - were expected in coming days, but he awaited confirmation.
The major powers would not participate directly in the "Syrian-owned, Syrian-led" constitutional effort, or the opening public ceremony, but they supported the process, he said.
"No one believes that the Constitutional Committee in itself will solve the conflict. But if it is understood as part of a broader political process, it could be a door-opener and a very important of course symbolic beginning of a political process," Pedersen said.
An agreement was reached under which the Assad regime, the Syrian opposition and civil society each submitted 50 members to serve on the panel. Each delegation includes Kurds, but there is no SDF or YPG representation, he said.
A group of 45 is to start drafting a document.
"I believe it is a historic occasion and of course for 150 Syrians to meet and to discuss a new Constitutional arrangement for their own country, that's a heavy responsibility," he said.
"In the end they will decide what kind of Constitution, or constitutional reform, will be necessary for Syria," he added.
Pedersen said that the two co-chairs were Ahmad Kuzbari for the Syrian government and Hadi AlBahra for the opposition.
"I think both the Astana 3 and the small group will hopefully express their continued full support to the constitutional process and the broader political process," he added.