Working without pay, service members are worried about feeding their families.
The Defense Department wants to scrutinize those with a 'foreign nexus.'
It was not immediately clear how many additional troops might be required.
The service is part of the U.S. military, but personnel are going unpaid because they are part of the Department of Homeland Security.
Coast Guardsmen are working without pay during the shutdown, and thousands of civilians working for the service are furloughed.
“Trust me, he’s heard every single argument on Afghanistan he could hear,” one official said.
The president has said he may leave Patrick Shanahan in an acting capacity “for a long time.”
President Trump called it "insane" that reports by the special inspector general for Afghanistan reconstruction are released to the public.
The barbs highlight challenges for new acting defense secretary Patrick Shanahan.
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