Foreign secretary says he has seen some proof that Assad's forces are preparing to launch chemical attack against rebels

William Hague has said he has seen "some evidence" that Bashar al-Assad's regime is preparing to use chemical weapons against Syrian rebels.

The foreign secretary would not give specific details of the intelligence, also seen by the US, but said it was enough to renew warnings to Assad that his 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, Mr 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 air strikes, Hague said the use of chemical weapons would be a "major change in situation".

Hague's words came amid claims from Russia, Syria's principal ally in the UN security council, that a series of leaks from the Pentagon and US state department about Assad's ability to deploy chemical weapons was being used by Obama to underpin threats of military action against his regime.

The claims have been met with incredulity by the Kremlin, which has suggested they are being used as a pretext to increase pressure on Assad and prepare for the use of force.

The US defence secretary, Leon Panetta, and the secretary of state, Hillary Clinton, have also stepped up warnings to Syria over its alleged stockpile of chemical weapons.

"I think there is no question that we remain very concerned," said Panetta. "Very concerned that as the opposition advances, in particular on Damascus, that the regime might very well consider the use of chemical weapons. The intelligence that we have causes serious concerns that this is being considered.

"The president of the United States has made very clear there will be consequences, there will be consequences if the Assad regime makes a terrible mistake by using these chemical weapons on their own people," Panetta added.

Clinton said Assad would cross "a red line" if he used chemical weapons. She said Washington was concerned that "an increasingly desperate Assad regime might turn to chemical weapons, or might lose control of them to one of the many groups that are now operating within Syria".