President Donald Trump's administration is sending military troops to secure the southern border as a caravan of thousands of Central American migrants moves north through Mexico.

Military Times reports that Defense Secretary Jim Mattis will send 800 active-duty troops to reinforce a contingent of 2,100 National Guardsmen who are already positioned at the U.S.-Mexico border. The additional forces will be responsible for "administrative duties," according to a defense official who spoke to the Times.

These troops will not be assisting border patrol agents with apprehending illegal immigrants. Instead, they will provide technical support at border crossing points, help provide fencing, and provide tents and medical care for law enforcement.

President Trump has vowed to "call up the U.S. Military and CLOSE OUR SOUTHERN BORDER" if Mexico is unable to stop a caravan of thousands of migrants from attempting to cross illegally into the United States.

The latest estimates are that some 4,000 people remain in the caravan, down from an estimated 7,000 after many dropped out due to sickness, police harassment, or fear from Trump's repeated warnings not to attempt illegal entry into the United States.