U.S. military troops stand on the U.S. side of the San Ysidro port of entry during a "large-scale operational readiness exercise" that briefly closed the border crossing last week. (Mario Tama/Getty Images)

The Trump administration is expected to extend the military mission along the U.S. border with Mexico, a White House official said Wednesday, likely keeping troops away from their normal posts through the holidays.

The official, speaking on the condition of anonymity to discuss plans that have not yet been announced, said the assignment was expected to last at least 45 days beyond its scheduled end date of Dec. 15.

That decision would affect almost 6,000 active duty military personnel now stationed in southern areas of Texas, Arizona and California, lengthening a mission critics have described as an unnecessary ploy to galvanize anti-immigrant sentiment.

News of a possible extension, which was first reported by NPR, comes days after a dramatic confrontation at a border crossing outside San Diego.

Border Patrol and Department of Homeland Security (DHS) officials this week defended authorities’ use of tear gas against migrants, including women and children, who tried to rush the border fence at California’s San Ysidro crossing on Sunday.

At least 5,000 migrants, many of them members of a caravan that has made its way through Mexico in recent weeks, are now waiting in Tijuana just across from San Diego to request asylum in the United States.

Previous administrations have sent National Guard troops to harden the border, but the use of large numbers of active duty military personnel, who face legal limits to their domestic activities, has no modern precedent. Critics have said the deployment undermines the military’s ability to prepare for foreign deployments.

Even before the incident at San Ysidro, the Pentagon had suggested an extension might be in the works.

“Some of those troops certainly will be home [for the holidays], I would anticipate they would be,” Defense Secretary Jim Mattis told reporters last week. “But some troops may not be or some new troops may be assigned to new missions . . . This is a dynamic situation.”

As of Wednesday, DHS has not asked the Pentagon to extend the timeline of its border mission, said Lt. Col. Jamie Davis, a Pentagon spokesman.

Homeland Security Secretary Kirstjen Nielsen, asked last week about the timetable for the troop deployment, said she had spoken to Mattis and had “no doubt” the Department of Defense would “continue to be our partners in this mission until it is resolved.”

A DHS official said the chaotic scenes along the border Sunday, when U.S. agents used tear gas to drive back stone-throwing crowds of migrants breaking through the fence, had made evident the need for a troop presence.

“What we saw Sunday is a clear indication of why we need [military] support,” the official said, also speaking on the condition of anonymity.

An announcement on the extension is expected as early as Friday. Last week, the White House issued an order that allows troops to use force to protect Border Patrol personnel and temporarily detain migrants.

The Pentagon has sought to tamp down speculation that troops could fire on migrants. Military police, who are typically armed and make up some of the forces deployed to the border, will bear primary responsible for protecting Border Patrol, but officials say they will not be armed if they are conducting crowd control activities.

“Only military police who are normally armed in their day-to-day duties will be carrying weapons, and would be involved in providing emergency backup support if the CBP requires assistance,” the U.S. Army North said in a statement.

Officials expect the extended mission will have a greater emphasis on protecting border personnel rather than the logistics and engineering activities, including stringing concertina wire and taking other steps to strengthen border, that dominated troops’ initial activities.

Dan Lamothe contributed to this report.