Syria warned on Saturday that rebels could use chemical weapons in their fight against President Bashar al-Assad's forces, and insisted that the government will never unleash such arms on its own people.

British Foreign Secretary William Hague, however, said there was evidence the Damascus government could actually employ chemical weapons stocks in the conflict which a rights group says has killed at least 42,000 people in nearly 21 months.

"Terrorist groups may resort to using chemical weapons against the Syrian people ... after having gained control of a toxic chlorine factory" east of Aleppo, the foreign ministry said, using the government term for rebel groups.