"Why are people leaving? Because they have lost hope after five years of endless conflict and they see only one winner, Daesh," Staffan de Mistura told journalists in Brussels, using another term for IS.
De Mistura said up to one million more people could be at risk in western Syria, potentially adding to the flood of refugees seeking safety in the European Union.
The international community was constantly trying but failing to come up with a formula but "there is no more time for such lengthy political processes," he said.
Some 11 million Syrians have been displaced from their homes by the conflict that began in March 2011, with four million becoming refugees.
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"It is time to find a solution ... Otherwise there will not be any Syrians left."
IS gains "should make everyone wake up; they are only a few miles (kilometres) from Damascus. Iran, Saudi Arabia, Turkey, Russia -- Daesh should be worrying them all," he added.
Earlier today, Moscow dismissed US concerns of a Russian military buildup in Syria, saying its military aid to long-time ally Assad was perfectly normal.
De Mistura declined to comment on the issue.