The claim may face strong behind the scenes diplomatic opposition due to the false claims by America and Britain that Saddam Hussein possessed weapons of mass destruction, which led to the invasion of Iraq by the UK and USA.
The CIA believes Syria possesses mustard gas and sarin, a deadly nerve agent. It is also thought to have attempted to develop more lethal and more persistent nerve agents, such as VX gas.
A report citing Turkish, Arab and Western intelligence agencies put Syria's stockpile at approximately 1000 tonnes of chemical weapons, stored in 50 towns and cities
Syria has not signed the Chemical Weapons Convention (CWC) or ratified the Biological and Toxin Weapons Convention (BTWC).
Speaking at a security conference in Bahrain, Hague said: "We and the US have seen some evidence and that is why we have issued strong warnings about it."
Pressed on what he had seen, Hague said: "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."
A Syrian official insisted last week that the regime 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.
Russia, Syria's principal ally in the UN security council, claimed 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 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, with Clinton saying 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".
l Meanwhile, Syrian rebel groups have elected Brigadier Selim Idris as president of the new unified military command.
Idris, a former officer from Assad's army who defected, will head the Islamist-dominated alliance. Also in the new military leadership is Jamal Marouf, an Islamist commander, and Ahmad al-Issa from al-Zawiya region in Idlib, as well as Colonel Abdelbasset al-Tawil, who has links with Salafists in the province.
Idris was elected by 30 military and civilian members of the joint command after talks attended by Western and Arab security officials in the Turkish city of Antalia.
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.