WASHINGTON — The Pentagon is sending about 3,000 more active-duty troops to the U.S.-Mexico border as President Donald Trump seeks to clamp down on illegal immigration and fulfill a central promise of his campaign, U.S. officials said Saturday.

His defense secretary, Pete Hegseth, has ordered elements of a Stryker brigade combat team and a general support aviation battalion for the mission, the Pentagon announced. The forces will arrive along the nearly 2,000-mile border in the coming weeks.

The Defense Department’s statement did not specify the size of the deployment, but it was put at about 3,000 by the officials, who were not authorized to discuss the matter publicly and spoke on condition of anonymity.

The Strykers are medium-armored wheeled personnel carriers.

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Already, about 9,200 U.S. troops in total are at the southern border, including 4,200 deployed under federal orders and about 5,000 National Guard troops under the control of governors.

The new troops will “reinforce and expand current border security operations to seal the border and protect the territorial integrity of the United States,” the Pentagon said.

Trump is determined to expand the military’s role in his effort to shut down the border and send detained migrants back to their home countries.

Military personnel have been sent to the border almost continuously since the 1990s to help address migration, drug trafficking and transnational crime.