Western and Arab powers agree with Syrian opposition leaders that President Bashar al-Assad should play no future role in government, but they are struggling to convince the rebels to attend key peace talks in Geneva next month.
The rebels, who met with the so-called Friends of Syria group of 11 countries in London on Tuesday, said they would not take part in the Geneva conference in late November if any regime members were there, and insisted that Assad's departure was essential.
But a defiant Assad still showed no sign of backing down after a two-and-a-half-year civil war that has left more than 115,000 people dead, saying that he was ready to run for re-election as president in 2014.
British Foreign Secretary William Hague said the London meeting had urged the Syrian National Coalition - the main opposition umbrella group - to "commit itself fully" to the so-called Geneva 2 talks.
Hague said the Friends of Syria agreed that they would put their "united and collective weight" behind efforts to form a transitional government and that "Assad would play no role in that future government of Syria".
US Secretary of State John Kerry took a similar position, saying that Assad had "lost all legitimacy".
But he too urged the opposition to go to Geneva, saying Syria was at risk of "implosion" if the civil war continued and said the only alternative to a negotiated settlement was "continued, if not increased killing".
The Syrian opposition is due to meet at the start of November to finalise their position on the Geneva talks, which are a follow-up to a peace conference held in the Swiss city in June 2012.
The head of the national coalition, Ahmad Jarba, appeared to be in no mood to compromise on his demands.
"There will not be negotiations at all without making sure that this meeting, Geneva 2, is basically to make sure there's a transitional period and for Assad to go," said Jarba, speaking in Arabic through a translator.
He added that there "must not be any people there that will represent the Syrian people except the group that is here".
Jarba also rejected hints by Hague and French Foreign Minister Laurent Fabius that Iran could play a role if it backs the plan for a transitional government.
The London talks grouped Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the US, together with Syrian opposition leaders.
Notably absent from the London's meeting is key Syria ally Russia, which has dismissed such gatherings in the past, saying they do not represent all Syrian people.
Hague and Kerry warned of the need to bolster moderate forces in Syria, where Islamist rebel groups linked to al-Qaeda are gaining strength at the expense of the more secular Free Syrian Army.
The US and Russia have been trying to organise the Geneva conference on the heels of the deal they reached for the destruction of Syria's chemical weapons in the wake of a deadly poison gas attack in August widely blamed on Damascus.
But Assad dealt an early blow to peace hopes, saying in an interview on Monday that the factors are not in place for the conference to succeed.
"No time has been set, and the factors are not yet in place if we want (Geneva 2) to succeed," Assad told Lebanon-based pan-Arab satellite channel Al-Mayadeen, adding that there was no guarantee about "which forces are taking part".
UN and Arab League mediator Lakhdar Brahimi will meet again in early November with US and Russian officials to prepare for the Geneva talks, the UN announced on Tuesday.
The meeting will be held on November 5 and will be followed by a meeting with the other three permanent members of the UN Security Council - France, Britain and China - according to UN spokesman Martin Nesirky.
Meanwhile Sigfrid Kaag, the chief of the United Nations and Organisation for the Prohibition of Chemical Weapons joint mission, said on Tuesday that the Syrian government had so far "fully co-operated" in destroying its massive chemical arsenal.
