Lakhdar Brahimi, who represents the UN and the Arab League, said that as many as 100,000 people could be killed in the next year as Syria moves toward rule by warlords.
Mr Brahimi has reported little progress in his mission to push forward a peace plan presented in June at a conference in Geneva.
The proposal calls for an open-ended ceasefire and the formation of a transitional government to run the country until elections can be held and a new constitution drafted.
The regime of president Bashar Assad and the scores of rebels groups fighting his forces across the country have shown no interest in negotiations.
The rebels' political leadership has called Assad's departure a prerequisite for any political solution, and it is unlikely that the opposition's National Coalition could even stop rebels from continuing to fight.
Likewise, it is doubtful that members of Assad's regime will voluntarily give up power.
Damascus has not commented officially on Mr Brahimi's plan, which he has pushed in the past week in meetings with Assad in Damascus, leading Russian officials in Moscow, and on Sunday with the head of the Arab League in Egypt.
Mr Brahimi said: "Peace and security in the world will be threatened directly from Syria if there is no solution within the next few months. The alternatives are a political solution or the full collapse of the Syrian state."
Since meeting Assad, he has given no indication how his plan was received.