U.N. Syria mediator Staffan de Mistura said there were "all the ingredients for a perfect storm".
The UN Security Council has met to discuss Syria and the likely offensive against the last rebel stronghold there, in Idlib province.
Three million people living in Idlib are at grave risk of suffering a humanitarian crisis if the fighting goes ahead.
U.N. Syria mediator Staffan de Mistura said there were "all the ingredients for a perfect storm."
"The dangers are profound that any battle for Idlib could be, would be, a horrific and bloody battle," he said.
Russia's ambassador to the UN said western powers were trying to protect the Islamic extremist insurgent groups based in Idlib.
"Unfortunately western capitals led by Washington seek to keep the regime of their extremist affiliates from falling and didn't use their influence to help the demarcation," Vassily Nebenzia said.
Meanwhile the US ambassador to the UN accused Syria of deliberately attacking civilians in its military operations.
"First they surround the civilian area. Next the make the preposterous claim that everyone in the area is a terrorist. So every man, woman and child becomes a target," Nikki Haley said.
"Then comes the "starve and surrender" campaign where they keep attacking the people until the people no longer have food, clean water or shelter. It's a playbook of death," she added.
A Syrian government offensive on Idlib could be the seven-year-long conflict's last decisive battle. After a long stalemate Iran and Russia entered the conflict and have helped Damascus regain most rebel-held territory.
Earlier on Friday the presidents of Turkey, Iran and Russia failed to agree on a ceasefire that would forestall the offensive.
Turkey's Tayyip Erdogan, Russia's Vladimir Putin and Iran's Hassan Rouhani, meeting in Tehran, agreed in a final statement that there could be no military solution to the conflict and it could only end through a negotiated political process.
But as Syrian government and Russian warplanes mounted air strikes in Idlib on Friday morning in a possible prelude to a full-scale offensive, Putin and Rouhani pushed back against Erdogan's call for a truce.
The Turkish leader said he feared a massacre and Turkey could not accommodate any more refugees flooding over its border.
In a series of tweets following the trilateral summit, Erdogan said Turkey will not "watch from the sidelines" any killings benefiting the Syrian agenda. Nor would he resort to methods that would disregard civilian lives that would "play into the hands of terrorists."
So far more than half a million people have been killed and 11 million have fled their homes, making Syria's the world's worst refugee crisis.