Syria's government and main political opposition have traded bitter accusations as a major peace conference Geneva II begins in Switzerland, BBC reports.
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 II document which lays out a political transition plan for Syria. But the key issue is President Assad's future.
It will hear from some 40 or so foreign ministers on Wednesday before direct Syrian talks begin in Geneva on Friday, and is 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.
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