Syrian regime forces shelled rebel and jihadist positions in the northwestern province of Idlib on Thursday and dropped leaflets urging people to surrender.
The province is the largest chunk of territory still in rebel hands, and President Bashar al-Assad has warned it would be his next priority.
The United Nations, for its part, appealed Thursday for talks to avert "a civilian bloodbath" in Idlib, on the border with Turkey.
"The war cannot be allowed to go to Idlib," the head of the United Nations humanitarian taskforce for Syria, Jan Egeland, told reporters in Geneva.
Egeland said he remained "hopeful" that diplomatic efforts could avert 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."
The warning came as government helicopters dropped leaflets over towns in Idlib's eastern countryside urging people to surrender, an AFP correspondent said.