The UN Special Envoy for Syria, Staffan de Mistura, sent out invitations to the delayed talks on Tuesday, without saying who had been invited or how many groups might participate.
Earlier, the opposition had cast doubt on whether it would go to Geneva, accusing the United States of adopting unacceptable Iranian and Russian ideas on solving the conflict.
A decision by the opposition's recently formed High Negotiations Committee (HNC) on whether to accept the invitation is due to be taken at a meeting in Riyadh.
"There is consensus in the High Committee on being positive in our decision," spokesman Salim al-Muslat told the Arabic news channel Arabiya al-Hadath. However, he added that the final decision would not be made at the meeting until Wednesday.
A Western diplomat said the Geneva talks could not be convened if the HNC were to stay away, and de Mistura would have to find a face-saving way to avoid the complete collapse of the process, perhaps by announcing a further delay.
The main Syrian Kurdish party said it had not been invited, a move some interpreted as intended to keep Turkey, which regards Syria's Kurdish fighters as terrorists, engaged in the process.
De Mistura has said the Geneva meeting will first seek a ceasefire and later work towards a political settlement. The talks are expected to last for six months, with diplomats shuttling between rival delegations in separate rooms.
The Syrian government, which is clawing back territory from the rebels with the help of Russian air strikes and other allies including Iranian fighters, has said it will attend.
The HNC has, however, repeatedly said the government and its allies must halt bombardments and lift blockades of besieged areas before it will join talks.
Opposition official Asaad al-Zoubi, who is due to head the HNC delegation, told Reuters that without the implementation of goodwill steps including the release of detainees "there will be no negotiations".
Reflecting opposition misgivings about the process, he told Al-Hadath that U.S. Secretary of State John Kerry had tabled Iranian and Russian ideas about Syria at a recent meeting with opposition leader Riad Hijab.
"It was not comfortable for us for America ... to adopt what came in the Iranian and Russian initiatives," Zoubi said.
The US Special Envoy for Syria, Michael Ratney, urged the opposition to attend.
"Our advice to the Syrian opposition is to take advantage of this opportunity to put the intentions of the regime to the test and to expose in front of international public opinion which are the parties serious in reaching a political settlement in Syria and which are not," he said.
The United States has supported the opposition to Assad, who it says has lost legitimacy and must leave power. But the opposition has been increasingly critical of American policy. Hijab said earlier this month the United States had backtracked on its position over Syria, softening its stance to accommodate Russia.
Diplomacy has repeatedly failed to resolve the conflict that has killed 250,000 people and forced millions from their homes, spawning a refugee crisis in neighbouring states and Europe. De Mistura is the third international envoy for Syria. His two predecessors - Kofi Annan and Lakhdar Brahimi - both quit.
