President Trump on Thursday warned the caravan of migrants heading to the southern U.S. border that they will not be let into the country and urged them to return home and apply for U.S. citizenship through the proper, legal channels.

"To those in the Caravan, turnaround, we are not letting people into the United States illegally. Go back to your Country and if you want, apply for citizenship like millions of others are doing!" Trump tweeted.

The caravan heading toward the border consists of Central American migrants. It started in Honduras earlier this month with a few hundred people, but it has grown substantially as the migrants have made their way north — some estimates put the numbers at higher than 10,000.

To support Immigration and Customs Enforcement, Defense Secretary Jim Mattis announced Thursday that he is sending 800 troops to the border after receiving a request Wednesday from the Department of Homeland Security.

The troops are not expected to use lethal force against caravan participants, but rather, they are likely to provide equipment and labor for tasks such as building fences. U.S. troops are not legally allowed to use deadly force in law enforcement operations.

U.S. and Mexican officials had previously reached an agreement on how to best deal with the caravan, but those plans fell apart after the migrants last week tore through border fences at the Guatemalan-Mexican border.