"The alternative is truly quite catastrophic, because we could see 400,000 people moving towards the Turkish border," Staffan de Mistura said after talks in Berlin with France and Germany's foreign ministers.
The Berlin talks, which also included Syria's main opposition leader Riad Hijab, is part of a week of whirlwind diplomacy as negotiators battle to salvage a collapsing truce.
De Mistura stressed that a return to peace talks hinged on a ceasefire.
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"The test is Aleppo now," he said.
German Foreign Minister Frank-Walter Steinmeier and France's Foreign Minister Jean-Marc Ayrault also issued similar warnings.
"If the ceasefire does not happen in the next few hours in Aleppo, everything would collapse," said Ayrault.
"All the hopes that we had put on a peace process would disappear, even though that's a necessity for Syrians for all the refugees who want to return to live in their country."