"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border," he tweeted. "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 White House has sought to make immigration the top issue of the Midterm elections, confident that Trump's hardline enforcement message will continue to drive his conservative base to the polls and even draw some crossover appeal among more moderate voters.
The President has latched on to the migrant caravan, helping draw attention to the group and labelling it a national security threat.
Pentagon officials and Homeland Security officials are preparing a joint news conference to describe the deployment in greater detail. A DHS official involved in the preparations said plans have yet to be finalised, but the troop levels that were in consideration last week were not realistic.
"We've asked for capabilities, and DHS is looking to fill capabilities, and the [Pentagon] is in the process of determining which units to send and how many personnel it'll take, and that has not yet been determined," said the official.
US officials say the border deployment under consideration would not include "trigger pullers" tasked with arresting migrants or other enforcement duties.
Rather, the troops would offer "logistical support" to the US Border Patrol and other Homeland Security agencies, and would include construction brigades, aerial transport crews and medical staff.
The larger deployment was first reported by the Wall Street Journal.
Immigrant rights groups have accused Trump of demagoguing the issue by inflating the size and security threat posed by the migrants, made up largely of families, including children.
The White House has put significant pressure on the Government of Mexico to block the caravan's advance. The group has diminished from a peak of nearly 7000 migrants, as some footsore travellers and parents with children have dropped out or fallen behind. At least 1000 caravan members have applied for asylum in Mexico, authorities say.
Mexican President Enrique Peña Nieto offered temporary work permits, medical care and other benefits to migrants if they agree to register with authorities and remain in the Mexican states of Chiapas and Oaxaca, far from the US border.
But the core group of mostly Honduran migrants, which authorities estimate at 3000 to 4000, has rejected his entreaty and continued heading north toward the US border.
The caravan remains at least 1450km from US territory, so its arrival is not imminent.
In an attempt to limit the caravan's size, Mexican police clashed with a smaller, separate group of Central American attempting to enter from Guatemala and catch up to the main group. At least one man was killed as police fired rubber bullets and tear gas.
Defence Secretary Jim Mattis said yesterday that he was finalising the details of what rules American troops deploying to the border would be operating under.
Mattis said he would make certain that whatever material was needed would get to the border, noting that what the military will be providing includes construction items such as jersey barriers. He said his staff had been meeting in recent days to determine how many personnel would go, but the deployment would be "phased."
"On the border, we are preparing what we call defence support for civilian authorities," Mattis said.
