UNITED NATIONS - Syria is stonewalling members of the global chemical weapons watchdog and refusing to seriously negotiate on the destruction of its facilities used to produce poison gas, the US envoy to the United Nations said on Wednesday.

The sharp criticism of the government of Syrian President Bashar Assad came after the Organization for the Prohibition of Chemical Weapons in The Hague said Syria has shipped about a third of its chemical weapons stockpile, including mustard gas, for destruction abroad.

"OPCW trying to reach agreement to destroy CW production facilities-#Syria refusing to seriously negotiate & is (about) to miss another deadline," US Ambassador Samantha Powers said on her Twitter feed.

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