Still, Lakhdar Brahimi told reporters at a news conference in Geneva that did not mean all hopes for a peace conference on Syria were dashed.
"(We) are still striving to see if we can have the conference before the end of the year," he said.
The diplomatic talks among world powers in Geneva at the UN's elegant Palais des Nations contrasted sharply with the heavy shelling and missile attacks being waged in a civil war that both sides still believe they might win militarily.
Assad's government signaled it was not ready to negotiate handing over power, while his main ally Russia insisted, once again, that pro-Assad Iran must be part of any talks on a war whose death count officially surpassed 100,000 more than three months ago.
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The United Nations, meanwhile, announced that as many as 40 per cent of Syrians now need humanitarian aid.
The Syrian war has left over 9 million citizens in need of humanitarian aid, including 6.5 million people who are now internally displaced, said Jens Laerke, a spokesman for the UN Office for the Coordination of Humanitarian Affairs.
Brahimi and Jeff Feltman, the UN's undersecretary-general for political affairs, met with senior Russian and US officials to see if a UN-sponsored peace conference bringing together Assad's government and a united opposition delegation could be convened this year.
The circle expanded for a second meeting with three permanent members of the UN's 15-nation Security Council Britain, France, China.
By late afternoon, it expanded again to include four of Syria's neighbors Turkey, Lebanon, Jordan and Iraq nations that are struggling to cope with a conflict that has produced more than 2.1 million Syrian refugees.