Syria’s government and main political opposition have traded bitter accusations as a major peace conference begins in Switzerland.
UN Secretary General Ban Ki-moon urged delegates to engage in constructive discussions, but neither side appeared prepared to abandon their positions.
US Secretary of State John Kerry said there was “no way possible” President Bashar al-Assad could remain in power.
The conflict has left more than 100,000 dead and millions displaced.
The summit is discussing the Geneva communique which lays out a political transition plan for Syria. But the key issue is President Assad’s future.
It will hear from about 40 foreign ministers on Wednesday before direct Syrian talks are scheduled to begin in Geneva on Friday.
This would be the first time the Syrian government and the main opposition – the National Coalition – have met face to face since the conflict began in 2011.
In an often angry opening speech, Syria’s Foreign Minister Walid Muallem said some states attending the talks had “Syrian blood on their hands” and were trying to destabilise the country.
“Syria the independent state will do all that is necessary to defend itself,” he said, and, addressing US Secretary of State John Kerry, said: “No-one in the world has the right to confer or withdraw the legitimacy of a president, a constitution or a law, except for the Syrians themselves.”
Mr Muallem ran far over the allotted 10-minute slot for each speaker, ignoring Mr Ban’s attempts to intervene.
“I have the right to give the Syrian version here,” he said.
The head of the the National Coalition, Ahmad Jarba, said it had not been the opposition’s choice to take up arms, but “was the choice imposed by the Syrian regime”.
He displayed a photograph taken from a report by three war crimes investigators which alleged “systematic” torture and execution of opposition detainees in Syria. The report was released on Tuesday but dismissed as not credible by Damascus.
Mr Jarba called on the government to immediately sign the Geneva document and transfer power to a transitional authority.
“For the Syrians, time is now blood.”
Opening the summit, Mr Ban urged all parties to engage “seriously and constructively” in the talks.
“Let me not mince words – the challenges before you and before all of us are formidable. But your presence here raises hope,” he added.
Russian Foreign Minister Sergei Lavrov said the talks “will not be simple, will not be quick”, but that there was “a historic responsibility on the shoulders of all participants”.
He also repeated his insistence that Iran, whose invitation to the summit was revoked, should be involved.
Mr Kerry used his opening remarks to remind the conference the uprising had begun as a peaceful process, but said the government had responded “with ever-increasing force”.
“We see only one option: negotiating a transition government born by mutual consent,” he said. “There is no way, no way possible, that a man who has led a brutal response to his own people can regain legitimacy to govern.”
‘Patience and persistence’International delegates to Geneva II have played down hopes of a breakthrough, saying the talks should be seen as the first step in a process.
German Foreign Minister Frank-Walter Steinmeier said: “We must have measured expectations. We will not see peace triumph during these discussions.”
Earlier this week the UN withdraw its invitation to Iran to attend the talks, saying it had orally accepted the Geneva Communique, the plan for a transitional Syrian governing body agreed at a UN-backed meeting in 2012, only to later fail to put the commitment in writing.
Iran’s President Hasan Rouhani said on Wednesday that the “lack of influential players in the meeting” meant he doubted its “success in fighting against terrorism… and its ability to resolve the Syria crisis”.