Russia, Turkey and Iran have failed to agree on a ceasefire that would prevent a Syrian government offensive in rebel-held Idlib.
The United Nations fears action could cause a humanitarian catastrophe in the city involving tens of thousands of civilians.
The UN envoy for Syria Staffan De Mistura says one way or another there needs to be a political end to the war “or we will see this war reach new levels of horrors”.
But Russian President Vladimir Putin and Iran’s President Hassan Rouhani pushed back against Turkey’s Recep Erdogan’s call for a truce.
The three leaders met in Tehran for a summit of key foreign players in Syria’s war.