International envoy Lakhdar Brahimi warned Sunday the Syrian war was worsening "by the day", AFP reports.
The comments were made as Brahimi announced a peace plan he believed could find support from world powers, including key Syria ally Russia.
The situation in Syria "is very bad and getting worse by the day," Brahimi told reporters in Cairo, a day after warning in Moscow that Damascus faced a choice between "hell or the political process."
He said he had crafted a ceasefire plan "that could be adopted by the international community."
"I have discussed this plan with Russia and Syria... I think this proposal could be adopted by the international community," the UN and Arab League envoy said, without giving details, according to AFP.
"There is a proposal for a political solution based on the Geneva declaration foreseeing a ceasefire, forming a government with complete prerogatives and a plan for parliamentary and presidential elections," he said, referring to a peace initiative that world powers agreed to in Geneva in June.
That plan was rejected by Syria's opposition, which is adamant that Assad's departure is a given before any national dialogue such as that under the Geneva initiative can take place.
Russia and China have so far vetoed three Security Council draft resolutions seeking to force Assad's hand with the threat of sanctions.
Brahimi held talks in Moscow on Saturday with Russian Foreign Minister Sergei Lavrov on his end-of-year bid to accelerate moves to halt the conflict that monitors say has killed 45,000 people.
The talks came amid signs that Russia was beginning to distance itself from Assad's government.
Meanwhile, reported AFP, Moscow dispatched a third naval vessel to the eastern Mediterranean on Sunday in readiness for a possible evacuation of Russian nationals, many of them women who married Syrian men during the Cold War years of close relations.
The Novocherkassk landing ship joined the Azov and Nikolai Filchenkov amphibious vessels already en route for Syria since Friday and is expected to dock in Tartus in the first 10 days of the new year, Russian news agencies reported.
The Tartus base is Russia's only remaining naval station outside the former Soviet Union and is seen as a major strategic asset for Moscow.
Russia has been accused of using the base to supply Assad's government with secret military shipments supplementing the official weapons sales that Moscow has made to Damascus since Soviet times.
Recent rebel gains prompted Russia to admit for the first time this month that Assad's days in power may be numbered.
On the ground, at least 100 people were killed in violence on Sunday, 43 of them civilians, according to a preliminary toll from the Syrian Observatory for Human Rights.