Approximately 5,600 active-duty troops are currently supporting border agents as part of a contentious mission ordered by President Donald Trump ahead of the mid-term elections in November.

The border deployment was supposed to wrap up by December 15, but due to an "ongoing threat" may now be extended until January 31, Department of Homeland Security (DHS) spokeswoman Katie Waldman said in a statement.

"This request refines support to ensure it remains aligned with the current situation, the nature of the mission, and CBP operational requirements," she said.

Pentagon spokesman Lieutenant Colonel Jamie Davis said the Defense Department was still reviewing the request and did not have an immediate response for DHS.

In addition to the 5,600 active-duty forces, another 2,100 or so National Guardsmen have been deployed to support operations at the border.

Their mission is to assist short-handed border patrol agents deal with the arrival of large groups of migrants in so-called "caravans."

Critics have called the deployment order a political stunt, coming as it did in the days ahead of the November 6 elections.

In the Mexican city of Tijuana, across the border from San Diego in California, more than 6,000 migrants are camped out hoping to apply for asylum, or sneak into the United States in some cases, fleeing poverty and violence in their home countries.

Much of the US troops' work so far has been to put up mile after mile of concertina and barbed-wire fencing along the border.