Kofi Annan, the joint UN-Arab League envoy to Syria, has proposed setting up a "unity government" to end the violence in the country, but several of Syria's main opposition groups have already rejected the plan.
Annan will discuss the plan in Geneva on Saturday with officials from nearly a dozen countries.
The proposal will call for multi-party elections, and an offer for "significant" international funding to rebuild war-torn Syria, according to a blueprint obtained by the AFP news agency.
"It is essential that any settlement provides for clear and irreversible steps in the transition according to a fixed time frame," the plan states.
Opposition groups have rejected the plan, saying they want Syrian President Bashar al-Assad banned from participating in any transitional government.
Annan's plan is vague on which Syrian officials would be excluded, saying only that it "could include members of the present government and the opposition and other groups, but would exclude from government those whose continued presence and participation would undermine the credibility of the transition and jeopardise stability and reconciliation".
"The proposal is still murky to us, but I can tell you that if it does not clearly state that Assad must step down, it will be unacceptable to us," said Samir Nashar, an executive member of the oppositon Syrian National Council.
The Syrian Muslim Brotherhood also rejected the plan, saying in an email statement that it was premature to negotiate a transitional government while violence was still ongoing.
"This is an attempt to circumvent the revolution of the Syrian people," the statement said.
'Decided by the Syrians'
The proposal is generally supported by the major powers - the US, Britain, France, China and Russia - who will attend Saturday's meeting, though Sergei Lavrov, the Russian foreign minister, warned that it should not "predetermine" Assad's fate.
"We do not support, and cannot support, any kind of meddling from outside, the imposition of recipes," he said at a news conference in Moscow.
"This applies to the fate of Bashar al-Assad. It, this fate, must be decided by the Syrians, the Syrian people themselves."
Assad himself delivered a similar message on Thursday night in an interview on Iranian state television.
"No non-Syrian model is acceptable because no one but us knows how to solve the problem," he said.
Foreign ministers from Turkey, Kuwait, Qatar and Iraq will also attend the meeting. Iran, Assad's other close ally, was excluded - a condition set down by the US - as was Saudi Arabia, which has supported the Syrian opposition.
Hillary Clinton, the US secretary of state, said on Wednesday that she had been in regular contact with Annan over his plan, and said it "embodies the principles needed for any political transition in Syria".
The latest diplomatic push comes amid worsening violence in Syria, including several recent attacks in and around the capital.
Two car bombs exploded on Thursday near the Palace of Justice in Damascus, Syria's highest court, though there were no immediate reports of any casualties.