WASHINGTON - US container ship Cape Ray began neutralizing chemical materials on Monday that were once part of Syrian President Bashar Assad's weapons arsenal, the Pentagon said.

The ship, in international waters in the Mediterranean Sea, began processing about 600 metric tonnes of chemical weapons and raw materials that could be used in them, Pentagon spokesman Colonel Steve Warren told reporters.

The US government has said the materials include mustard gas and components for the nerve agent sarin.

The materials were transferred to the Cape Ray earlier in July from a vessel that brought them out of Syria. Assad agreed last year to turn over his chemical weapons stockpile as part of a disarmament deal.

Warren said it is expected that the ship will need 60 days to process the materials.

The Cape Ray is equipped with a hydrolysis system that uses substances like water, sodium hydroxide and sodium hypochlorite to make chemical warfare agents safe enough to be disposed of at commercial sites.

Warren said the processed material would be transferred to Finland and Germany.

Washington and Moscow supported the agreement under which Assad surrendered Syria's chemical weapons after it was accused of using them against civilians.

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