Defense Secretary Jim Mattis is expected to sign an order sending at least 800 troops to the U.S.-Mexico border, where a caravan of migrants is heading north from Central America.

The Department of Homeland Security requested the troops, NPR's Tom Bowman reports, after President Trump said he is intent on stopping any of the migrants from crossing the border.

"We're told they're going to be military police and engineers," Bowman says, "to assist Border Patrol, not to take part in any sort of law enforcement activities."

The troops are part of a security operation that is expected to be up and running by Oct. 30. This week, members of the caravan were making their way through a border crossing in southern Mexico, crossing from Guatemala into Ciudad Hidalgo near the Pacific Coast — roughly 1,100 miles from the U.S. border, via the most direct route to Brownsville, Texas.

The service personnel sent by the Pentagon will be either active duty or reserve troops, Bowman adds.

Immigration and customs agents in the area have already been bolstered by some 2,100 National Guard troops, as the Trump administration has made it a priority to prevent undocumented immigrants from entering the U.S.

As it has headed north, the migrant caravan has grown to include thousands of people. Many of them are from Honduras, where the movement originated; others have come from neighboring countries, such as El Salvador, Guatemala and Nicaragua.

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