Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE has approved the extension of the deployment of active duty troops to the southern U.S. border through Jan. 31, a spokesperson for the Pentagon told The Hill Tuesday.

"The Secretary of Defense has approved an extension of the ongoing Department of Defense (DoD) support to the Department of Homeland Security (DHS) response to migrant caravan arrivals. DoD support to DHS is authorized until Jan. 31, 2019," the spokesperson said.

The spokesperson did not say how many of the 5,600 troops currently deployed to the border would stay.

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Two U.S. officials told ABC News last week that an extension could use a rotational deployment, shifting different units through the border support mission.

A portion of the troops currently deployed to the border were expected to be relieved on Dec. 15, when the mission was originally set to end.

The relief would allow those troops to go home for the holidays.

Last week, The Associated Press reported that the Department of Homeland Security requested that 4,000 U.S. troops remain at the border through the end of January.

Caravans of thousands of immigrants, which President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE has characterized as an "invasion," have moved into northern Mexico, just south of the U.S. border.

Two weeks ago, roughly a thousand immigrants tried to breach the border illegally, with some throwing rocks, until border guards dispersed them with tear gas.

Monday night, dozens of immigrants who were camped in the Mexican border city of Tijuana breached the U.S.-Mexico border.

The military deployed to the southern border cannot legally act in a law enforcement capacity but provides logistical support to the border guards stationed at heavily trafficked areas.

-Ellen Mitchell contributed.

-Updated 5:41 p.m.