Lakhdar Brahimi said the state would collapse without a solution, reiterating warnings the country could turn into "hell" and a new Somalia.
"I say the solution must be this year: 2013, and, God willing, before the second anniversary of this crisis," Mr Brahimi, joint UN-Arab League envoy, said at the Arab League in Cairo, referring to the start of the uprising in March 2011.
"People are talking about Syria being split into a number of small states. This is not what will happen, what will happen is Somalisation: warlords," he said. Somalia has been without effective central government since civil war broke out there in 1991.
Mr Brahimi, referring to the Geneva plan, said: "There are sound foundations to build a peace process through which the Syrians themselves can end the war and fighting and to build the future."
The plan included a ceasefire, the formation of a government and steps towards elections, either for a new president, or a new parliament. It left the fate of President Bashar al Assad unclear although the Syrian opposition and foreign governments who back it insist he must go.