Lead negotiator for the main opposition High Negotiations Committee (HNC), Mohamad Sabra, said in an interview today that the UN-backed talks in Geneva remained "stalled".
"There can be no real and viable political solution without the presence of the Americans," he said.
The United States has a "moral duty" to throw its weight behind efforts to end the six-year conflict, he added.
Years of diplomatic initiatives have failed to end the war, which has killed more than 320,000 people and displaced millions since it started in March 2011 with protests against President Bashar al-Assad's regime.
Two UN-backed taskforces co-chaired by the United States and Russia and aimed at securing ceasefires and access for humanitarian aid in Syria seem to be having less impact.
At the same time, regime supporters Russia and Iran along with rebel-backer Turkey have been pushing separate negotiations in Astana since January after gains on the ground by Damascus turned the tables in the conflict.
Russian President Vladimir Putin was today set to host his Iranian counterpart Hassan Rouhani for talks that are being closely watched for signs of their next moves in Syria.
While urging Washington to be more fully engaged in the peace process, Sabra warned that a deal would be impossible "if the Russians do not withdraw their (military) support for the regime."
Russia's entry into the conflict in September 2015 -- when it began launching air strikes to shore up Assad's forces -- played a significant role in turning the tide for the Syrian regime.
The HNC delegation was expected to meet with Russian Deputy Foreign Minister Genady Gatilov tomorrow, after he met with the government delegation today.
"Until now, we don't have a partner in these negotiations," he insisted.
His comments came amid a fifth round of negotiations in Geneva being mediated by UN Special Envoy Staffan de Mistura which have remained clouded by persistent violence on the ground and deadlock over the country's political future.
The government has ruled out discussing Assad's possible departure, while the HNC says it will refuse any deal that leaves him in power.
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