WASHINGTON (AP) -- President Donald Trump is signing a proclamation directing the departments of Defense and Homeland Security to work together with governors to deploy the National Guard to the U.S.-Mexico border.

Homeland Security Secretary Kirstjen Nielsen made the announcement at the White House on Wednesday. She says details are still being worked out, but National Guard personnel are expected to assist with U.S. Customs and Border Protection's mission.

Nielsen said that DHS and the Pentagon will be working closely with governors in the affected states. She says that deployment will be done as expeditiously as possible and that Guard troops could begin heading to the border as soon as Wednesday night.

Trump announced on Tuesday that he plans to deploy the military to the southern border until his long-promised wall is built.

The president has been frustrated by Congress' refusal to fund building a wall along the length of the U.S. border as well as an increase in illegal border crossings.

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