United Nations Secretary-General Antonio Guterres has appealed to Russia, Iran and Turkey to "spare no effort to find solutions that protect civilians" in Syria's Idlib and says it is "absolutely essential" a full-scale battle is avoided.
"This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict," he told reporters on Tuesday.
An estimated three million people live in Idlib - the last major stronghold of active opposition to President Bashar al-Assad. The UN has described it as a "dumping ground" for people evacuated and displaced from elsewhere in Syria during the seven-year war.
Assad has vowed to retake the region, backed by his Russian and Iranian allies. Syrian government and Russian warplanes began air strikes in Idlib last week in a possible prelude to a full-scale offensive. They say they are targeting terrorist groups.
"I understand that the present situation in Idlib is not sustainable and the presence of terrorist groups cannot be tolerated. But fighting terrorism does not absolve warring parties of their core obligations under international law," said Guterres.
Senior officials from Russia, Iran and Turkey met with UN Syria envoy Staffan de Mistura in Geneva on Tuesday. The presidents of Turkey, Iran and Russia met in Tehran on Friday but failed to agree on a ceasefire in Idlib.
Russia briefed the 15-member UN Security Council on Tuesday on the Tehran meeting, which was held as part of the so-called Astana process.
"Russia, Iran and Assad are demolishing Idlib and asking us to call it peace. But here's the reality: Astana has failed. It has failed to stop the violence or to promote a political solution," US Ambassador to the UN Nikki Haley said.
Australian Associated Press