The United States, France and Britain have threatened to respond if the head of the Syrian regime, Bashar 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.
The US, France and the UK 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.
"Our position on the Assad regime's use of chemical weapons is unchanged," the three countries said in their 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."
John Bolton, US President Donald Trump's national security adviser, also warned that Washington would respond "very strongly" if Assad uses chemical weapons in the offensive on Idlib, the biggest area in rebel-held hands, which borders Turkey.
"Just so there's no confusion here, if the Syrian regime uses chemical weapons we will respond very strongly and they really ought to think about this a long time," Bolton told a press conference during a visit to Jerusalem.
