UN Secretary-General Antonio Guterres warned Syria and its backers Tuesday against launching a full-scale offensive in Idlib, saying the rebel-held province "must not be transformed into a bloodbath."
"It is absolutely essential to avoid a full-scale battle in Idlib," Guterres told reporters at UN headquarters.
"This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict."
The UN chief spoke after Russia and Iran clashed with Turkey at a summit last week on plans for military action to restore Syria's control over Idlib, where three million people live.
Turkey is appealing for a ceasefire to allow for negotiations on the fate of armed groups in Idlib, Syria's last major rebel stronghold, but its call has been rejected by Russia and Iran.
Guterres said he was appealing to Iran, Russia and Turkey to "spare no effort to find solutions that protect civilians" and warned that any use of chemical weapons would be "totally unacceptable."
The UN Security Council met earlier to hear a briefing from Russia, which insisted that the planned assault on Idlib is a "counterterrorism operation" and that measures will be taken to spare civilians.
"There is no doubt that an all-out military operation would result in a major humanitarian catastrophe," Turkish Ambassador Feridun Sinirlioglu told the council meeting.
Air strikes and bombings will trigger a "massive wave of refugees and tremendous security risks for Turkey, the rest of Europe and beyond," he warned.
Turkey, which has sent troops to Idlib and supports some of the armed groups, called for "an immediate ceasefire" and urged "the international community to vocally and actively support our efforts to this end."
Iran, Russia and Turkey last year set up the Astana process, a negotiating track to end Syria's war that has largely eclipsed the UN-led peace process.