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.
Despite Syrian denials, experts from the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW) have said they are confident government forces were behind an attack involving the nerve agent Sarin on a rebel-held town in southern Idlib in April 2017 that killed more than 80 people.
Mr Jeffrey said a “major diplomatic initiative” was now needed to end the seven-year civil war.
He said there was “a new commitment” by President Donald Trump to remain involved in Syria until the Islamic State group (IS) was defeated and to ensure that Iranian fighters – who are supporting the Syrian authorities in the conflict – leave the country.
Mr Jeffrey said President Assad had “no future as a ruler” in Syria, but it was not Washington’s job to oust him. He said the US would work with Russia on a political transition.
There are believed to be up to 30,000 rebel and jihadist fighters in Idlib.
The UN says the region is home to some 2.9 million people, including a million children.
More than half of the civilians have already been displaced at least once from elsewhere in Syria and have nowhere left to go.
UN officials say as many as 800,000 people could be displaced and that the already high number of people in need of aid could increase dramatically.