Defense Secretary Jim Mattis is sending 800 troops to the U.S.-Mexican border to give support to border agents after receiving a request Wednesday from the Department of Homeland Security, according to multiple reports.

The Pentagon will decide which troops will be dispatched to the southern border once the request is officially approved. The troops, which are expected to comprise mainly engineers and doctors, are likely to provide equipment and labor for tasks such as building fences.

U.S. troops are not legally permitted to use lethal force in law enforcement operations such as at the U.S.-Mexican border.

DHS Secretary Kirstjen Nielsen will publicly address the request later in the day Thursday.

The troops will most likely be in place by next week as the U.S. braces for a caravan of more than 5,000 migrants to reach the southern border.

The group of Central American migrants, which originated in Honduras earlier this month, is more than 1,000 miles from the U.S.-Mexican border, but could reach it as soon as next week if they were to travel by bus or train the rest of the way.

[Trump: 'Blame the Democrats' for assault on the southern border]

The U.S. and Mexico reached a deal earlier on how to deal with the caravan, but efforts have proven ineffective at halting the caravan as they tore through border fences at the Guatemalan-Mexican border last week and charged into Mexico.

President Trump previously threatened to use military forces if he had to in order to stop the flow of illegal immigration into the U.S. by way of Mexico.