FILE - Defense Secretary Jim Mattis waits for Chinese Minister of Defense General Wei Fenghe to begin an arrival ceremony at the Pentagon, Nov. 9, 2018.
FILE - Defense Secretary Jim Mattis waits for Chinese Minister of Defense General Wei Fenghe to begin an arrival ceremony at the Pentagon, Nov. 9, 2018.

PENTAGON - Secretary of Defense Jim Mattis says he will travel to the U.S. southern border Wednesday, where thousands of American troops have been deployed to support border patrol agents.

Speaking to reporters Tuesday at the Pentagon, Mattis said there has been "no change" to the U.S. military mission "at this time."

"We're doing the obstacle emplacement, we do have helicopters down there," he said. "We will have to see what the future holds, but right now that is the only mission I have."

In this photo provided by the U.S. Air Force, sold
FILE - In this photo provided by the U.S. Air Force, soldiers from the 89th Military Police Brigade, and 41st Engineering Company, 19th Engineering Battalion, Fort Riley, Kansas, arrive at Valley International Airport in Harlingen, Texas, Nov. 1, 2018.

President Donald Trump called for increased border security to prevent a caravan of asylum-seeking migrants from Central America from entering the United States, declaring on several occasions its approach constituted a "national emergency."

WATCH: Border troops

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More than 5,600 active-duty troops and another 2,100 National Guard troops have moved into the U.S.-Mexico border area due to requests for help from the Department of Homeland Security.

A U.S. defense official told VOA that Mattis will visit McAllen, Texas, where about 1,000 U.S. troops are deployed.

The U.S. troops assigned to help with border security are legally prohibited from engaging in domestic law enforcement, such as arresting migrants crossing the border.

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Mattis' border visit comes as sources say Trump is considering the removal of Department of Homeland Security Secretary Kirstjen Nielsen.

Nielsen was selected as secretary at Homeland Security after John Kelly left the post to become Trump's chief of staff.