Turkey on Tuesday appealed for international backing for a ceasefire in Idlib, telling the UN Security Council that an all-out assault on Syria's rebel-held province will trigger a massive wave of refugees and could threaten Europe.
Russia called for the council meeting to brief members on a summit it held with Iran and Turkey on military plans to re-take Idlib, the last major rebel stronghold in Syria.
"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."
Russia and Iran rejected Turkey's call for a ceasefire at the summit in Tehran on Friday.
US Ambassador Nikki Haley said Syrian forces, backed by Russia and Iran, had already launched 100 air strikes on Idlib this month and that their sole aim was "a bloody military conquest of Idlib."
Haley warned that "the consequences will be dire" if the assault goes ahead and there are mass casualties. "The world will hold them responsible," she added.
Britain and France backed the call for a ceasefire, recalling that these had been agreed under arrangements for a "de-escalation zone" in Idlib that were overseen by Russia, Iran and Turkey.
Russia and Iran insisted that a military offensive in Idlib would be a counter-terrorism operation and that measures will be taken to spare civilians.
"We cannot allow terrorists to hold hostage and use hundreds of thousands of people of Idlib as human shields," said Russian Ambassador Vassily Nebenzia.
The Security Council was meeting just days after hearing UN peace envoy Staffan de Mistura warn of a looming catastrophe from the military offensive in Idlib, where some three million people live.
UN diplomats are hoping there is still time for talks to avoid large-scale violence in Idlib.