UN Secretary-General Ban Ki-moon says that the 20-month conflict in Syria has reached "new and appalling heights of brutality and violence."
Ban and international mediator Lakhdar Brahimi addressed the UN General Assembly on the revolt against President Bashar al-Assad.
The fighting has killed an estimated 40,000 people, mostly civilians. Ban said he was "horrified and saddened" by the relentless killing.
Ban said he expected the number of refugees -- currently about 480,000 -- to reach 700,000 by early next year.
Brahimi told the General Assembly that the conflict had spread and intensified greatly recently.
"Military confrontations in Syria have continued unabated. Indeed, fighting has expanded geographically to almost all parts of Syria and intensified very significantly," Brahimi said.
"Antigovernment forces have reportedly achieved gains on the ground in several areas this past few weeks."
Brahimi also said that Syria was in danger of becoming a failed state, and called for the UN Security Council to approve a resolution providing for a large peacekeeping mission.
"Any peace process must include necessarily a binding agreement on the cessation of all forms of violence. As I said earlier, there is no trust between the parties and for fighting to stop a strong, well planned observation system must be put in place," he said.
"Such observation can best be organized through a large, robust peacekeeping force and, naturally, that cannot be envisaged without a Security Council resolution."
U.S. State Department spokeswoman Victoria Nuland said that Brahimi was due to meet later on November 30 with Secretary of State Hillary Clinton.
"We're interested to hear how he sees the situation," Nuland said. "He's had a number of consultations, we will have a chance also to talk to him about our evolving relationship with the Syrian Opposition Council, as well."
Violence Near Airport
Government warplanes, meanwhile, bombed rebel targets on November 30 close to the Damascus airport road.
The London-based Syrian Observatory for Human Rights said the road from the capital to Damascus airport had reopened, a day after fighting during which a bus carrying airport employees was hit by a shell, killing two people.
The United Nations said four of its staffers were also wounded when their bus was targeted by gunfire on its way to the airport on November 29.
Reports quoting unnamed security sources also said that, in the town of Tal Kalakh on the border with Lebanon, 12 young Sunni volunteers from the Lebanese city of Tripoli were killed on November 30.
The violence has partially halted international flights. The Internet and most telephone lines also were down for a second day, in an unprecedented outage.
Meanwhile, in Tokyo, delegates from some 60 countries gathered for to look into ways to step up pressure on Assad.
Japanese Foreign Minister Koichiro Gemba told the Friends of Syria group that the international community had to act together.
With reporting by Reuters and AFP