The presidents of Iran, Russia and Turkey are holding a key summit in Tehran that could decide the fate of Syria’s last major rebel stronghold.
There are fears of a humanitarian disaster if, as expected, a large-scale battle breaks out in Idlib.
Fresh air strikes on rebel positions were reported on Friday morning.
Iran, Russia and Turkey have played central roles in the Syrian conflict.
Turkey – which has long backed rebel groups – fears an all-out assault will trigger another huge refugee crisis on its southern border.
Russia and Iran – which have provided vital support for Syrian President Bashar al-Assad – believe the rebels in Idlib must be wiped out.
Russian planes have bombed rebel areas in the north-western region as Syrian government troops mass for the expected offensive.
The US envoy, Jim Jeffrey, said the anticipated conflict would be a “reckless escalation”, Reuters reported.