US President Barack Obama warned Syrian strongman Bashar al-Assad on Monday not to make the "tragic mistake" of turning to his stockpile of chemical weapons.
Assad's beleaguered regime had earlier threatened to unleash the weapons if Syria faced international military intervention, although it vowed not to turn them against its own civilians.
"Given the regime's stockpile of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching," Obama told an audience of US veterans in the western state of Nevada.
"They will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," he said.
Earlier, Syrian Foreign Ministry spokesperson Jihad Makdissi had warned Syria could use chemical weapons if attacked by outsiders, although he back-tracked later to clarify that he was not confirming Damascus has such arms.
"Syria will not use any chemical or other unconventional weapons against its civilians, and will only use them in case of external aggression," he said.
Syria is in the grip of a 16-month-long conflict triggered by Assad's brutal repression of a pro-democracy revolt. Western and Arab powers have called for him to step down and allow an orderly transition of power.