The United States on Wednesday urged Syria and its backers to halt plans for an all-out military attack against rebel-held Idlib as the UN Security Council prepared to address the crisis.
"With millions of civilians at risk, an offensive against Idlib would be a reckless escalation," US Ambassador Nikki Haley said in a statement.
"The regime and its backers must stop their military campaign in all its forms to allow the UN-led political process to have a chance to succeed."
The council will discuss the crisis in Idlib on Friday after Secretary-General Antonio Guterres warned of a "humanitarian catastrophe" in the province of 3 million people if an all-out military assault is launched.
On Tuesday, Russian warplanes resumed air strikes on Idlib after a 22-day pause, pressing ahead with a campaign it has described as aimed at rooting out Al-Qaeda-linked groups.
At a news conference on Tuesday, Haley suggested that Russia and Syria could move ahead with a counter-terrorism operation but must spare civilians.
"This is a tragic situation, and if they want to continue to go the route of taking over Syria, they can do that," Haley said.
"But they cannot do it with chemical weapons. They can't do it assaulting their people."
Ten countries, all non-permanent council members, separately expressed "deep concern" in the event of a full-scale military operation in Idlib and urged the Syrian government "to prioritize the protection of civilians."
UN envoy Staffan de Mistura is to hold talks next week in Geneva with Russia and Iran, Syria's main backers, and Turkey, which supports some opposition groups.
More than 350,000 people have died in Syria's seven-year war, but UN officials fear the assault on Idlib could trigger one of the worst bloodbaths of the conflict.