The UN peace envoy for Syria on Friday proposed a plan for Al-Qaeda-linked fighters in Syria's Idlib province to pull back from civilian areas to avert a bloodbath in the rebel-held province.
The United Nations has warned of a humanitarian disaster if Syrian forces, backed by Russia and Iran, launch an all-out attack in Idlib, the last major rebel bastion.
Addressing the Security Council, Staffan de Mistura also called for evacuation corridors to be opened for civilians to voluntarily leave the war zone, under UN monitoring.
The council was meeting as the presidents of Iran, Russia and Turkey agreed during a summit in Tehran to work in a "spirit of cooperation" to stabilize the situation in Idlib.
The three countries are guarantors of the Astana process, a track of talks on Syria's war launched after Russia's 2015 military intervention that has largely eclipsed the UN peace process in Geneva.
De Mistura proposed that a deadline be set for all fighters -- in particular the Al-Qaeda-linked Nusra Front -- to move away from populated areas and that no military attacks would be launched during the pullback.
"This would apply in particular for Al-Nusra, who should be notified by the guarantors, in particular Turkey, which still has the capacity to send messages," he said.
Russia told the council meeting that some 40 to 45 armed groups with up to 50,000 people were operating in Idlib. Some 16,000 are fighting in the ranks of Al-Nusra and 11,300 others are members of hardline groups, said Russian Ambassador Vassily Nebenzia.
Syrian civilians are "worn out by extremist tyranny" in Idlib, said Nebenzia, adding that terrorist groups remain "a legitimate target for liquidation."
Some three million people live in the northwest region, which came under renewed air strikes and bombings earlier this week.
"There are more babies in Idlib than there are terrorists," said British Ambassador Karen Pierce. "I think this should give those engaged in military action pause for thought."
Britain recalled that targeting civilians amounts to war crimes and warned that Syrian commanders and military units loyal to President Bashar al-Assad will be held accountable for any violations in Idlib.
US Ambassador Nikki Haley warned: "If Assad, Russia, and Iran continue, the consequences will be dire."
De Mistura is to hold talks next week with the three guarantors in Geneva next week.
More than 350,000 people have been killed in Syria's seven-year war, but UN diplomats fear the assault on Idlib could trigger one of the worst bloodbaths of the conflict.