Libya's stock of chemical weapons, including over 10 tons of mustard gas, are under strict US surveillance, the Pentagon said Tuesday. "We have been and continue to monitor chemical weapons sites," spokesperson Colonel David Lapan said. Moammar Qaddafi''s regime joined the Organization for the Prohibition of Chemical Weapons in 2004 after renouncing weapons of mass destruction in December 2003, but still had to eliminate 11.25 tons of mustard gas when the uprising to remove him from power began in February. All 3,563 munitions -- such as bombs, shells and missiles -- that could serve as a carrier to distribute mustard gas have, however, been destroyed, according to the OPCW. Mustard gas depots are located at Rabta, about 100 kilometers south of Tripoli and at the Ruwagha site in Al-Jufra, south of Sirte. Under rules of the Chemical Weapons Convention, which Libya signed onto, such plants must remain closed or destroyed. Mustard gas, also known as sulfur mustards, can form large blisters or chemical burns on exposed skin and eyes, as well as cause bleeding and blistering within the respiratory system.