In a letter to Guterres, the groups, including a number of opposition local councils, said envoy Staffan de Mistura attended the congress even as the Syrian regime continued to show little interest in UN-led peace talks.
![Lori and George Schappell shared 30% brain [Shutterstock]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Lori and George Schappell shared 30% brain [Shutterstock]")
"Instead of an onus on Russia to get the regime to the table in a meaningful way before Russia was rewarded, you gave them the reward of UN legitimacy upfront," said the letter released Thursday.
Syria's main opposition coalition and Kurdish groups boycotted the congress held this week, just days after a ninth round of UN-led talks in Vienna failed to yield progress.
The 1,400 delegates at the congress agreed to set up a committee to draft a post-war constitution.
The opposition fears that the new constitution will pave the way to allowing Syrian President Bashar al-Assad to stay in power despite six years of bloodshed.
UN spokesman Stephane Dujarric said that Russia had provided assurances that the result of the congress would be "a contribution" to the UN-led peace talks in Geneva.
The West views Russia's peace efforts in Syria with suspicion, concerned that Moscow is seeking to sideline the UN talks.
A Syrian government source said Thursday that the congress was "the cornerstone of the political process and solid base upon which dialogue will be launched from now on."
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![Lori and George Schappell shared 30% brain [Shutterstock]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Lori and George Schappell shared 30% brain [Shutterstock]")
