"The war cannot be allowed to go to Idlib," the head of the United Nations humanitarian task force for Syria, Jan Egeland, told reporters in Geneva.
President Bashar al-Assad has warned that the northwestern province would be his military's next priority. Artillery reportedly slammed into rebel and jihadist positions in the area earlier on Thursday.
Egeland said he remained "hopeful" that diplomatic efforts underway could stave off a major ground offensive that would force hundreds of thousands to flee.
"It is bad now," in Idlib, Egeland said. "It could be 100 times worse."
Idlib is home to about 2.5 million people, up to half of whom are rebels and civilians transferred en masse from other territory that fell to Syrian troops after intense assaults.
A major military operation in Idlib would pose a humanitarian nightmare as there is no opposition territory left in Syria where people could be evacuated to, Egeland said.
"I cannot see evacuations to other opposition-controlled areas," he said, explaining that contingency plans were being formed to deal with a range of scenarios.
At Thursday's humanitarian task force meeting, ambassadors discussed options to ramp up assistance in the event of additional massive displacement, Egeland said, noting that "it is very hard to take on more mouths to feed."