The Syrian Negotiation Commission (SNC) warned on Wednesday that UN-brokered peace talks in Geneva could be undermined by a “parallel process” in the Russian city of Sochi.
The previous eight rounds of talks stalled over the fate of Syrian president Bashar Al Assad, with regime negotiators refusing to meet the opposition unless it drop demands that Mr Al Assad leave office.
“The SNC called for international pressure on the regime to negotiate in Geneva,” said Yahya Aridi, the spokesperson for the opposition's unified delegation.
"The Sochi process risks undermining the Geneva talks and the goal of transition set out in the Geneva communique and UN Security Council Resolution 2254,” Mr Aridi said.
After meeting with UN secretary general Antonio Guterres in New York, opposition leaders headed to Washington on Tuesday to shore up the Geneva talks, which are expected to resume on January 21.
"There is a political process, it is in Geneva, and it is not working because the regime refuses to negotiate. A parallel process will not change that reality and in fact risks emboldening the regime and its commitment to a military strategy,” Mr Aridi said.
The unified Syrian delegation is hoping to persuade Washington to throw its weight behind UN-brokered peace talks in Geneva in an effort to counter Russia's bid to set up a parallel process in Sochi.
The US is "the most important player in the Syrian fight, the only state that can make a balance with the Russian influence," Nasr Hariri, head of the unified delegation said.
Western countries view the Sochi peace talks with scepticism, concerned that Russia and Iran will carve out a settlement that will favour their own ally Mr Al Assad.
The UN has yet to decide whether it will take part in the Sochi conference, a move that would bring credibility to the Russian-led effort.
For its part, added Mr Hariri, the Syrian opposition has not taken a "final decision" as to whether they would boycott the Sochi talks.
Meanwhile, the opposition delegation urged Matthew Rycroft, the United Kingdom's UN representative, to come up with a strategy to "better support Geneva and apply real pressure to the regime and its backers."
"The SNC was clear that the international community, including the UK, needed to help change the dynamic by working with allies to put pressure on the regime to negotiate at Geneva," Mr Aridi said.
Russia's ambassador to the UN Vasily Nebenzya told reporters on Tuesday that Moscow wants the Sochi talks to "reinvigorate" and contribute to the UN-led peace efforts in Geneva.
He expressed hope that a new round of Geneva talks would be "more fruitful, and that it will contribute to Sochi, and then Sochi in reciprocity will contribute to future Genevas."