British Foreign Secretary William Hague says the UK and the US 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.
That statement followed a warning on Monday from US President Barack Obama that there would be consequences if Syrian 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 the interview by the BBC's Frank Gardner, 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.
Mr Hague, speaking while at a security conference in Bahrain, 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.
They say contingency plans have been drawn up if they show signs of being readied to be loaded and used as weapons.
Meanwhile, Syrian rebel fighters are maintaining their offensive against Damascus airport, which they say is a "fair target".
Rebel spokesmen say the airport is being used by the Syrian military and that it should be avoided by civilians.
There has been fierce fighting in recent weeks in the countryside around Damascus, known as the Ghouta.
The city's international airport has been inaccessible or closed to civilian flights repeatedly over the past two weeks.