After meeting with UN Secretary-General Antonio Guterres in New York, opposition leaders are heading to Washington on Tuesday for meetings to shore up the Geneva talks, which are expected to resume on January 21.
"All of us, all of our countries are waiting for an American role" in finding a settlement to the nearly seven-year war, Nasr Hariri told AFP in an interview.
The United States is "the most important player in the Syrian fight, the only state that can make a balance with the Russian influence," he said.
The delegation led by Hariri hopes to meet with US national security adviser H.R McMaster, State Department officials and members of Congress to make their case for US engagement.
The opposition's appeal to the US administration comes as Russia is asking the United Nations to take part in a peace congress it is hosting along with Iran in Sochi on January 29 and 30.
Western countries view the Sochi peace talks with skepticism, concerned that Russia and Iran will carve out a settlement that will favor their ally Syrian President Bashar al-Assad.
Hariri said he was told by Guterres that no decision had been made yet on whether the United Nations would take part in the Sochi conference, a move that would bring credibility to the Russian-led effort.
For its part, the Syrian opposition has not taken a "final decision" on whether to boycott the Sochi talks, said Hariri, who became the opposition's chief negotiator in November.
- Invigorate the Geneva talks -
Russia's UN Ambassador Vassily Nebenzia told reporters that he still hopes the United Nations will attend the Sochi conference, which he said would help "re-invigorate" the Geneva talks.
"We think that it's in the interest of (the) UN to participate," said Nebenzia, stressing that the conference was not a "stand-alone event" but one that should contribute to kickstart the UN talks.
Eight rounds of talks in Geneva have repeatedly stumbled over Assad's fate, with negotiators from Damascus refusing to meet the opposition directly until it drops demands that he leave office.
Assad's forces have regained the upper hand in the war, re-taking territory in a campaign backed by Russia's military intervention.
Without Washington as an "active political player in Geneva, I think the game will continue, by wasting time, establishing parallel tracks to hijack the political process in Geneva to Sochi, to Astana," said Hariri.
President Donald Trump's administration has shown little interest in the diplomatic effort to end the war in Syria, focusing instead on defeating the Islamic State (IS) group.
After the US-led coalition drove IS out of Raqa, Washington must now step up its involvement in forging a peace agreement to keep IS at bay and to prevent Iran from expanding its influence, said Hariri.
More than 340,000 people have died in Syria's nearly seven-year war and over half of the population has been driven from their homes.