William Hague has revealed that he had seen "some evidence" that Syria is preparing to use chemical weapons against rebels.
The Foreign Secretary declined to give details of the intelligence, but again warned that Bashar Assad's regime would face action if they were deployed.
American satellites and other tools have reportedly detected increased activity at several chemical weapons depots in Syria.
At least one military base is also said to have been ordered to begin combining components of Sarin nerve gas to make it ready to use.
The Syrian regime has denied any plans to use chemical weapons against it own people.
Speaking to the BBC at a security conference in the Gulf on Saturday morning, Hague was asked whether he had seen proof that Syria was preparing such weapons.
"We have seen some evidence of that," he said. "We and the US, as I said in parliament this week, have seen some evidence of that and that is why we have issued strong warnings about it. We have done so directly to the Syrian regime."
Pressed on what kind of evidence he had seen, Hague replied: "We absolutely cannot be specific about that because clearly those are intelligence sources that these things come from.
"But we have seen enough evidence to know that they need a warning and they have received that warning."
Amid speculation that the regime could be targeted with airstrikes, Hague said the use of chemical weapons would be a "major change in situation".