Russia, Iran and Turkey announced on Friday that a Syrian congress of national dialogue will be held in the Russian city of Sochi on January 29-30.
The United Nations meanwhile urged them to support the struggling Geneva peace process.
Moscow, Tehran and Ankara announced the date in a joint statement after talks in the Kazakh capital, Astana, which also involved the Damascus government and some opposition groups.
But the trio has yet to agree a list of participants - Turkey has earlier objected to the presence of the main Syrian Kurdish group - and confirm their participation. Some rebels said they had not yet made up their mind.
UN Syria envoy Staffan de Mistura, who also attended the Astana talks, said that Russia’s plan to convene the congress should be assessed by its ability to contribute to and support the UN-led Geneva talks on ending the war in Syria.
Opposition groups present at the talks sided with him.
“As for the Sochi conference…the United Nations assesses any (such) initiative through its role and its help in achieving peace,” the rebel delegation said.
“We told the Russians that Sochi will not be an alternative to Geneva, and we want to end the misery of the Syrian people and let humanitarian aid in,” Ahmed Tohme, the head of the delegation, said in the statement.