The opposition Syrian High Negotiations Committee (HNC) will not attend the Syrian National Dialogue Congress that is scheduled to be held in the Russian Black Sea resort of Sochi next week, a spokesman said on Saturday.
Yahya al-Aridi, the spokesman for the opposition delegation in Vienna, was speaking at the end of two days of UN-brokered talks between the Syrian regime and opposition.
"This whole round in Vienna was supposed to be a crucial one, a test for commitment. And we didn't see this commitment. And the UN didn't see this commitment," said Aridi.
"It's quite clear that somebody there is obstructing the whole process and wanting to sideline the importance of Geneva, the political process as a whole," Aridi added.
There have been eight previous rounds of failed UN-backed talks in Geneva.
"Russia has not succeeded in promoting its conference," the HNC said on its Twitter account.
"The HNC has decided not to participate at Sochi after marathon negotiations with the UN and representatives of countries involved in Syria."
UN Syria envoy Staffan de Mistura, speaking to reporters early Saturday, admitted there had been a disheartening lack of progress up until now in finding a solution for the Syrian war.
He stressed the legitimacy of the UN-led talks over Russia's parallel peace push, however, saying firmly that a political transition for Syria "is to be reached in the UN-led Geneva process".
"I hope that the forthcoming Syrian national dialogue congress in Sochi will contribute to a revived and credible intra-Syrian process under the UN in Geneva," he added.
The UN has yet to decide whether to attend the congress.
"The secretary general, Antonio Guterres, is being briefed and will be briefed tonight on the outcome of these Vienna discussions. Then it will be up to him to take the decision what will be the UN response to the invitation to attend Sochi," de Mistura added.