Defense Secretary James Mattis plans to sign an order sending at least 800 troops to the southern border in response to the caravan of thousands of migrants headed for the United States, according to reports Thursday.

The deployment will give a boost to some 2,000 National Guard troops already assisting at the border under a previous Pentagon agreement.

The extra active-duty troops will provide logistical support, including vehicles, tents and equipment.

The news came just hours after President Trump — who in April directed Mattis to send the National Guard to the border — again vowed that he’s “bringing out the military for this national emergency.”

“They will be stopped!” Trump tweeted Thursday morning.

Later in the day, he offered a message directly to the Central American marchers, who are about 1,000 miles away from the US.

“To those in the Caravan, turnaround, [sic] we are not letting people into the United States illegally,” the president tweeted.

“Go back to your Country and if you want, apply for citizenship like millions of others are doing!”

Mexican officials say nearly 1,700 people have dropped out of the caravan to apply for asylum in Mexico. A few hundred, meanwhile, have accepted government offers to get on buses and return to their home countries.

The crowd of migrants, which stretched more than a mile long, continued trekking through Mexico Thursday after camping overnight in the southern Mexican town of Mapastepec.

They’ve covered 95 miles, mostly on foot, and amid stifling heat since bursting through a metal gate at Mexico’s southernmost border last week.

Earlier this week, the United Nations estimated the caravan was as large as 7,200 people.

But the Mexican government gave its own figure Wednesday of “approximately 3,630.”

Many of the migrants are fleeing violence and poverty in Honduras, Guatemala, El Salvador and Nicaragua.

Trump on Thursday boasted about the US taking a “hard line on illegal immigration” and praised Italian Prime Minister Giuseppe Conte for doing the same.

“Just spoke with Prime Minister @GiuseppeConteIT of Italy concerning many subjects, including the fact that Italy is now taking a very hard line on illegal immigration . . .” Trump tweeted.

“I agree with their stance 100%, and the United States is likewise taking a very hard line on illegal immigration. The Prime Minister is working very hard on the economy of Italy — he will be successful!”

With Wires