Washington (CNN) The Pentagon has rejected a State Department request to deploy a detachment of US Marines to Taiwan to help safeguard the de facto American embassy there, three US officials tell CNN.

The decision to deny the request was made by Secretary of Defense James Mattis last month, according to two of the officials.

One defense official told CNN that prior to the request being rejected, military planners had anticipated the Marines arriving in Taiwan as early as next month.

Asked about the decision, a State Department official told CNN "we do not discuss specific security matters concerning the protection of our facilities or personnel."

US officials told CNN in June that the State Department had sought the deployment of a Marine Security Guard detachment to help bolster security at the new American Institute in Taiwan facility which will house some 450 staff and sits in the outskirts of Taiwan's capital, Taipei.

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