The Defense Department says it's sending 5,200 active duty troops to "harden" the southern border against a migrant caravan making its way through Mexico.
He added: "This is an invasion of our Country and our Military is waiting for you!"
Trump in recent weeks has repeatedly said more troops are needed to tighten security at the border, and he has made political capital of the caravan ahead of important mid-term congressional elections that could see the Democrats regain a degree of power.
His warning comes as the United States military is reportedly preparing to send 5,000 troops to the border, a significant increase over the initial estimates of 800 reported last week. About 600 migrants in the second group tried to cross a bridge from Guatemala to Mexico en masse Monday.
Many in the caravan have been on the road for more than two weeks, since the group first formed in San Pedro Sula, Honduras.Raul Medina Melendez, security chief for the tiny municipality in Oaxaca state, said the town was distributing sandwiches and water to migrants camped in the central square Saturday night when a man with a megaphone asked people to wait their turn.Some hurled insults at the man with the megaphone, then they attacked him, Medina said.
"If they apply for asylum, we are going to hold them until such time as their trial takes place".
General Terrence O'Shaughnessy, the head of U.S. Northern Command, defended the operation at a briefing on Tuesday.
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Kevin McAleenan, the US commissioner of Customs and Border Protection, estimated on Monday that the caravan was comprised of about 3,500 migrants.
"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border", Trump wrote without providing evidence, doubling down on the hardline anti-immigrant rhetoric that helped fuel his 2016 election victory.
Trump denied his focus on the caravan is meant to help Republicans in next week's midterms, saying, "This has nothing to do with elections". But there already is a bottleneck of would-be asylum seekers waiting at some US border crossings to make their claims, some waiting as long as five weeks. The troops expected to be sent to the border will primarily consist of military police and engineers, but the military also reportedly plans to send Marines. The Pentagon says an additional 5,200 active-duty troops could join them.
Trump said the United States would build "tent cities" to house migrants seeking asylum, rather than releasing them while they await court decisions.
The U.S. military has already begun delivering jersey barriers to the southern border in conjunction with the deployment plans. Although the troops will be armed, they are not constitutionally allowed to enforce immigration law, and are to be used only to provide logistical and other kinds of support to the Border Patrol - bolstering the approximately 2,000 National Guard soldiers already on the border.
The caravan remains at least 900 miles from USA territory, so its arrival is not imminent.