The US secretary of state has urged all sides to attend next week's talks in Switzerland aimed at ending the conflict in Syria.
John Kerry, speaking in Washington DC on Thursday, said it was imperative that the main Syrian opposition vote on Friday to be part of the discussions which begin on Wednesday in Montreux.
He said any individual chosen to lead a political transition in Syria must be acceptable to both the government and opposing forces.
The opposition groups say they should lead to a transitional government that would see Syrian President Bashar al-Assad step down and end four decades of his family's rule in Syria.
The government rejects the demand and says Assad may run for re-election.
The US, which backs the opposition, and Russia, Assad's main arms supplier, have pushed Syria's warring parties to meet at the talks under the auspices of the UN.
|Analysis: Scepticism and hope before Syria peace talks
More than 120,000 have been killed in the ongoing Syrian conflict.
Fighting between al-Qaeda-linked fighters and more moderate opposition groups has added another layer of complexity to the nearly three-year conflict that the rebels have been waging against the government.
Kerry's remarks comes as a leaked letter obtained by Al Jazeera appears to indicate that the Assad government has agreed to attend the peace conference.
However, the letter - from Walid al-Muallem, Syria's foreign minister, to Ban Ki-moon, the UN secretary-general - appears to set conditions for the peace talks.
"It should be noted that we do not agree with certain points mentioned in the letter of invitation, simply for the reason that they are in conflict with the legal and political position of the State of Syria," Muallem writes in reply to an invitation from Ban.
Muallem goes on to say: "It remains a priority for the Syrian people to continue to fight terrorism ...
"We demand the countries supporting terrorism cease and refrain from funding, training, arming or harbouring terrorist groups in harmony with international law and UN resolutions."
Reacting to the revelation, the main umbrella Syrian opposition body in exile, the National Coalition, said the letter undermines the peace talks, adding that the Assad regime needs to make it absolutely clear that it accepts the so-called Geneva 2 road map.
In a related development, Syria's centrist internal opposition group, which rejects the armed rebellion against the Assad regime, said on Thursday that it would not attend the Montreux conference.
Khaled Dahowd, an executive member of the National Coordination Body (NCB), accused Russia and the US of rushing the conference to promote their own interests in the region, rather than those of the Syrian people.
The NCB has tense relations with the National Coalition, which is rife with divisions.
Lakhdar Brahimi, UN special envoy to Syria, said in a statement that he deeply regretted but respected the NCB's decision.
In his remarks on Thursday, Kerry criticised the Assad government, saying that it was to blame for the instability that had attracted fighters from around the world to Syria.
"We too are deeply concerned about the rise of extremism," he said.
Kerry said the aim of the peace conference is to establish a process needed to form by mutual consent a transitional governmental body with full executive powers.
Any names submitted for leadership of Syria's transition must be agreed to by both the opposition and the Syrian government, he said.
"This means that any figure that is deemed unacceptable by either side, whether President Assad or a member of the opposition, cannot be a part of the future," he said.
Earlier on Thurday, a senior Syrian government official said the peace talks - known informally as "Geneva II" although it is being held in Montreux - will not solve the Syrian crisis.
Speaking at a seminar in the capital Damascus, Ali Haider, national reconciliation minister, said: "Don't expect anything from Geneva II. Neither Geneva II, not Geneva III nor Geneva X will solve the Syrian crisis.
"The solution has begun, and will continue through the military triumph of the state ... and through the staying power and resilience of the state and all its institutions, in the face of its enemies who were betting on its collapse."