As part of the move to extend the active duty troops' mission past a January 31 deadline, the Pentagon said it was "transitioning its support at the southwestern border from hardening ports of entry to mobile surveillance and detection, as well as concertina wire emplacement between ports of entry."

"DoD will continue to provide aviation support," the statement added, referring to the Department of Defense.

The Pentagon already has about 2,350 active-duty troops stationed along the border, deployed under a controversial order President Donald Trump gave last year ahead of midterm elections.

Additionally, about 2,200 National Guardsmen are supporting border operations.

The troops' role had primarily been to erect miles (kilometers) of concertina-wire fencing along popular crossing points.

Trump is unwavering on his demands for a border fence, triggering a major political stalemate and a partial government shutdown that is entering its fourth week, with mounting associated costs.

The president only recently backed away from talk of an emergency declaration, after pressing it for days as a way out of the continuing budget standoff.