GENEVA - 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 face-to-face meeting between the Syrian government and the main opposition - the National Coalition - since the conflict began in 2011.
The BBC's Paul Wood, in Montreux, says there were some extraordinarily ill-tempered scenes and some very direct language as the conference got under way.
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.
Addressing US Secretary of State John Kerry, he 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.
"You live in New York. I live in Syria," Mr Muallem told Mr Ban. "I have the right to give the Syrian version here. After three years of suffering, this is my right."
The US state department condemned Mr Muallem's remarks as "inflammatory rhetoric", and urged the government to take "real, concrete steps to increase humanitarian access and improve the lives of the people suffering the most".
The head of the National Coalition, Ahmad Jarba, said in his speech 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.