President Trump is expected to extend the deployment of thousands of U.S. troops to the U.S.-Mexico border into January rather than withdrawing the personnel in the middle of December, Pentagon officials tell NPR. The move would further extend the rare deployment of active-duty troops at the Mexico border, rather than just National Guard personnel.

The deployment orders for active-duty troops had been scheduled to end on Dec. 15. But defense officials tell NPR that the much of the active-duty troop deployment is expected to be extended into January and that part of the force may also be switched out for new troops.

News that the deployment will likely extend through the holidays comes days after U.S. agents fired tear gas at migrants who tried to cross into the United States from Tijuana, Mexico, on Sunday. That event took place at the San Ysidro Land Port of Entry between Tijuana and San Diego — "the busiest land border crossing in the Western Hemisphere," as NPR reported.

Earlier this month, Defense Secretary Jim Mattis said the troops are meant to support Customs and Border Protection agents and provide logistical help — everything from installing barbed wire to moving personnel to where they're needed. The troops include military engineers and military police, who are supposed to leave any actual law enforcement duties to CBP officials.

Some of the active-duty military personnel are being sent to the border without weapons, as Steve Walsh of member station KPBS reported last week, during a visit to Marines and military police along a fence at the line between the U.S. and Mexico. Armed members of the force guard the Marines as they work, Walsh reported.

The military deployment came after the Department of Homeland Security asked the Pentagon to help its Border Patrol units along the southern border in October, planning for the arrival of a caravan of thousands of migrants from Central America. Trump's political opponents called the move, which came just before the November midterm elections, was called an overreaction — and even a stunt.

In addition to the 5,900 active-duty personnel, the Trump administration has sent 2,100 National Guard troops to the border.

When Mattis visited the deployed troops the week before Thanksgiving, he said, "I think that it's very clear that support [for] the border police — or Border Patrol is necessary right now." But he did not go into detail about why the use of active-duty personnel was required.

The president has called the caravan's arrival an invasion. But as of earlier this month, Mattis and others were not using those kinds of terms to describe the situation.

In April, Trump signed a proclamation to send National Guard troops along the southern border, as part of his campaign to cut down on illegal immigration — a key part of his political platform.

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