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.
Earlier, the new US envoy for Syria said there was “evidence” that Syrian government forces were preparing to use chemical weapons.
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.
“I am very sure that we have very, very good grounds to be making these warnings,” Mr Jeffrey said in his first interview since being appointed.
“Any offensive is to us objectionable as a reckless escalation. There is lots of evidence that chemical weapons are being prepared.”
He did not give details of the evidence he was referring to.
The Syrian government has repeatedly denied ever using chemical weapons.
The US state department warned on Monday that Washington would respond to any chemical attacks by the Syrian government or its allies. -BBC
