The United States has offered to destroy Syria's chemical weapons on an American ship, and is looking for a suitable Mediterranean port where the operation can be carried out, the world's chemical weapons watchdog has said.

The Netherlands-based Organization for the Prohibition of Chemical Weapons (OPCW) said on Saturday that the weapons would be destroyed on a U.S. vessel at sea using hydrolysis, and that a ship was already undergoing modifications to carry out the operation.

OPCW spokesman, Michael Luhan, declined to name the vessel to be used.

The operation would destroy 500 tons of Syria's most dangerous chemical weapons, including nerve agents, which are considered too dangerous to import into a country or process commercially.

Known as "priority chemical weapons", these weapons have to be out of Syria by December 31 under an international deal agreed to avert military strikes on Damascus.

Earlier this month, the OPCW adopted a final roadmap for ridding Syria of more than 1,000 tons of dangerous chemicals by mid-2014.