Opposition sources said it appeared increasingly unlikely the talks would open in the Swiss city tomorrow as planned.
The Saudi-backed High Negotiations Committee - formed last month in an effort to unite Syria's fractious opposition - said it was waiting for answers from the United Nations before agreeing to attend the talks.
Western diplomats have piled pressure on the opposition to participate in the negotiations, part of the biggest push yet to resolve Syria's nearly five-year civil war.
The Committee has asked for "clarifications" after the UN issued invitations to other opposition figures and wants assurances from the international community that it will move to end regime attacks on civilians and allow humanitarian aid.
Salem al-Meslet, a Committee spokesman, said it was waiting for an answer from UN Secretary General Ban Ki-moon on the application of a Security Council resolution adopted in December that endorsed a roadmap for peace.
Security Council members "must take their responsibilities and commit to applying resolution 2254. We are waiting for an answer," Meslet said.
"We are serious about participating in the negotiations. The ones who are hindering the start (of talks) are those who are bombing and starving civilians."
France-based Middle East analyst Agnes Levallois said the opposition was growing increasingly frustrated that the question of President Bashar al-Assad's fate was being put off. "Assad is feeling stronger and stronger so is being inflexible. He wants to be sure there are no longer talks on his future so is putting forward the humanitarian issue," she said.
