Currently, about 2,350 active-duty troops are working along the border, where they are helping border patrol agents by providing logistical support and installing concertina-wire fencing.

They were deployed under an order President Donald Trump gave before midterm elections in November as "caravans" of Central American migrants made their way to the border to seek asylum from violence and poverty in their own countries.

At a Pentagon press conference, Shanahan said "several thousand" additional troops would deploy on the border mission, which was supposed to wrap up January 31 but has been extended through September.

The Department of Homeland Security has asked for additional help putting up fencing and with "expanded surveillance," Shanahan said.

Pentagon spokesman Lieutenant Colonel Jamie Davis later said the military is currently determining which units will be involved, and that there would be an increase of "a few thousand" troops.

"We will provide more clarity on the numbers when we have it," Davis said.

The border troop deployment was one of several issues on which Trump and former defense secretary Jim Mattis disagreed before he quit in December after Trump's shock decision to pull troops from Syria.

Shanahan, who had been deputy defense secretary, succeeded Mattis on January 1.