The United States, France and Britain on Tuesday threatened to respond if Syrian President Bashar al-Assad uses chemical weapons in its offensive to retake Idlib province.
In a joint statement, the three powers said they were "gravely concerned" over the military offensive in Idlib and the resulting humanitarian consequences.
"We also underline our concern at the potential for further – and illegal – use of chemical weapons," they said.
"We remain resolved to act if the Assad regime uses chemical weapons again."
The three UN Security Council powers released the joint statement to mark the fifth anniversary of the sarin attack in Ghouta that killed more than 300 people.
That attack, which the West blamed on Assad's forces, led to a US-Russian agreement to rid Syria of its chemical stockpile and its means to produce the deadly chemicals.
"Our position on the Assad regime's use of chemical weapons is unchanged," said the statement.
"As we have demonstrated, we will respond appropriately to any further use of chemical weapons by the Syrian regime, which has had such devastating humanitarian consequences for the Syrian population."
The United States, France and Britain in April launched airstrikes on Syrian targets in response to a chemical weapons attack in the town of Douma that left scores dead.
The one-night operation hit three sites in Syria amid warnings from Russia that a military intervention could lead to a broader war.
After seven years of war, Assad has set his sights on retaking control of the Idlib, the biggest area in rebel-held hands, which borders Turkey.
The regime holds the southeastern tip of the province that is home to some 2.5 million people -- more than half displaced by Syria's war or bused into Idlib under surrender deals.
The council is scheduled to discuss the humanitarian situation in Syria next week.