The US has ordered the deployment of active duty military forces to the Mexican border to install surveillance cameras and reinforce existing barriers with concertina wire, Under Secretary of Defence for Policy John Rood disclosed in Congressional testimony on 29 January.

The Daily Beast reported that Secretary of Defence Patrick M. Shanahan noted that the Pentagon is already preparing to send "several thousand" more troops to the border replying to a US Department of Homeland Security call for more help on monitoring and "laying down more concertina wire".

"The president would declare national emergency and then we would do the planning and do the recommendation on how the military could support the request", Shanahan stated, as quoted by The Daily Beast.

It would likely involve more troops and possibly extra aviation to help stop illegal crossings, according to media reports.

"It's really around this mission of monitoring, surveilling and detection. If we detect activity, that's when we notify Customs and Border Protection, and they do their apprehension", Shanahan said.

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The statement comes after Undersecretary of Defence for Policy John Rood told Congress on 29 January that US officials are tracking three new migrant caravans from Central America that are making their way to the US-Mexican border, the biggest of which has more than 12,000 would-be immigrants.

Just over 2,300 active duty troops are presently deployed, Rood said. He did not disclose the size of the latest deployment, which was triggered in part by a CPB request for an additional 150 miles of concertina wire by the 31st of March. National Guard troops have also been deployed on the border.

Rood told lawmakers to expect additional deployments this year in response to developments such as additional migrant caravans from Central America.

President Donald Trump has threatened to declare a national emergency, which would allow military forces to build a border barrier, despite opposition by Democratic lawmakers.