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

But the mission, under which troops assist civilian border patrol agents by providing logistical support and installing concertina-wire fencing, has been described by opponents as a political stunt designed to create the illusion of a crisis.

"The Department of Defense will deploy approximately 3,750 additional US forces to provide the additional support to CBP (Customs and Border Protection) at the southwest border that Acting Secretary of Defense (Pat) Shanahan approved Jan. 11," a statement said.

This additional deployment is for three months, the statement added, and will bring to 4,350 the total number of troops at the border.

Their tasks will include "a mobile surveillance capability through the end of September 2019, as well as the emplacement of approximately 150 miles of concertina wire between ports of entry."

The news comes ahead of a February 15 deadline that President Donald Trump set for Congress to agree on funding for building a border wall which he made a key pledge in his election campaign.

The president says that existing sections of walls should be extended along the border to stop what he describes as an uncontrolled invasion of criminals.

Trump underscored these claims Sunday. "With Caravans marching through Mexico and toward our Country, Republicans must be prepared to do whatever is necessary for STRONG Border Security," he tweeted.

"Dems do nothing. If there is no Wall, there is no Security. Human Trafficking, Drugs and Criminals of all dimensions - KEEP OUT!"

Democrats, who control the lower house, have repeatedly rejected Trump's funding demands, saying that he has made the wall project a political crusade to demonize immigrants and to satisfy his base.

He previously tried to pressure Congress into backing his idea by refusing to sign off on budgets for swathes of the federal government, leading to a five-week shutdown of some 800,000 government jobs.

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.