About 1,000 additional personnel will join the roughly 5,000 already operating on the border, in a return to the peak force size deployed shortly after the mission began last November.
The troops currently stationed at the border have also been ordered to string more concertina wire and install detection systems in remote areas away from official ports of entry, the senior defence official said.
The official, who was authorised by the Pentagon to speak to reporters only on the condition of anonymity, said that so far active-duty military forces had installed 70 miles of concertina wire, reinforced ports of entry, provided medical support to migrants and helped transport Customs and Border Protection (CBP) agents by aircraft.
“That mission has evolved,” the senior defence official said.
“We are now transitioning to supporting (areas) between the ports of entry. We're laying down another 140 miles of concertina wire – we're about 30 per cent done with that – as well as providing a ground-based detection and monitoring mission in support of CBP.”
CBP agents use unattended ground sensors to detect and monitor migrants who come across the border illegally.
The new influx of troops brings the deployment size back near its high of about 5,900 last November.
It comes as Donald Trump prepares to take $6.1bn from the Pentagon budget, without authorisation from Congress, to fund construction of a border wall. He is using a combination of emergency powers and counter-drug authorities to do so.
In addition to the active-duty forces, another roughly 2,300 members of the National Guard are deployed to the border to help relieve what the Trump administration has described as strain on CBP due to a large number of Central American families crossing the border.
The governors of California and New Mexico ordered the withdrawal of most of the guardsmen from the border in their states, calling the deployment political theatre. As a result, the overall number of guardsmen deployed could soon decrease.
House Speaker Nancy Pelosi, said on Friday that lawmakers plan to vote on Tuesday on a measure rejecting Mr Trump's national emergency declaration. Democrats anticipate they will have some Republican support in the House and the Senate.
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