President Trump told Fox News on Monday evening that Central American migrants who are approaching the U.S.-Mexico border in caravans are "wasting their time" and vowed, "they are not coming in."

Trump spoke to "The Ingraham Angle" hours after the Pentagon announced it would deploy some 5,200 troops to the southern border in what the commander of U.S. Northern Command described as an effort to "harden the southern border" by stiffening defenses at and near legal entry points.

"When they are captured, we don't let them out," Trump told host Laura Ingraham. "We're not letting them out ... We're not catching, we're not releasing ... We're not letting them into this country."

Trump also said the administration would build "tent cities" to house any asylum-seekers who have joined the caravan.

"We're going to put tents up all over the place," the president said. "We're not going to build structures and spend ... hundreds of millions of dollars. We're going to have tents, they're going to be very nice and they're going to wait and if they don't get asylum, they get out."

The president repeatedly has raised the alarm about the first migrant caravan approaching the U.S., which contains an estimated 4,000 people. A second caravan, with an estimated 600 migrants, has been picking up steam and marked by violence. About 600 migrants in the second group tried to cross a bridge from Guatemala to Mexico en masse Monday. The riverbank standoff with Mexico police followed a more violent confrontation Sunday when the migrants used sticks and rocks against officers. One migrant was killed Sunday night by a head wound, but the cause was unclear.

TRUMP EMBRACES 'NATIONALIST' LABEL, REJECTS BLAME OVER HATE-FILLED ATTACKS

"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border," Trump tweeted Monday morning. "Please go back, you will not be admitted into the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!"

The forces headed to the U.S.-Mexico border are support troops – not combat troops – and will include three engineering battalions with "heavy equipment", as well as aviation and medical personnel. Most of the troops are on active duty and will carry weapons, but will not be part of any law enforcement effort in deference to the Posse Comitatus Act, which prohibits the federal government from using the armed forces in a domestic police role.

THOUSANDS OF U.S. TROOPS DEPLOYING TO SOUTHERN BORDER

The White House is also weighing additional border security measures, including blocking those traveling in the caravan from seeking legal asylum and preventing them from entering the United States.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.