British Foreign Secretary William Hague says Britain and the United States have seen evidence that Syria is preparing to use chemical weapons.
Mr Hague told the BBC there was "enough evidence to know that they need a warning".
The Foreign Secretary did not give details, as he said the evidence had come from "intelligence sources".
A Syrian official insisted last week that it would "never, under any circumstances" use such weapons.
President Barack Obama warned on Monday that there would be consequences if President Bashar al-Assad used chemical weapons against his people.
Mr Hague told the BBC: "The president of the United States warned of serious consequences and he means it."
Pressed in an interview by the BBC's security correspondent, he said he could understand why the public might be sceptical after the blunders made over Iraq's alleged weapons of mass destruction 10 years ago.
At a security conference in Bahrain, Mr Hague said the use of chemical weapons would be a "major change" and said he hoped that Syria would heed the warnings.
Western military sources in the Gulf have told the BBC that Syria's chemical weapons are concentrated at five air bases and are being closely watched.
Syria is believed to possess mustard gas and sarin, a highly toxic nerve agent.