The United States promises to "respond swiftly and appropriately" if Syria uses chemical weapons in Idlib province, the last rebel stronghold in Syria.
The White House said Tuesday it is "closely monitoring" the fate of Idlib, where it said "millions of innocent civilians are under threat of an imminent ... attack" by the regime of President Bashar al-Assad.
It said President Donald Trump "has warned that such an attack would be a reckless escalation of an already tragic conflict."
The White House said it "will continue to work tirelessly with its allies to find a lasting diplomatic solution to resolve the hostilities in Syria" under a United Nations Security Council resolution