US officials told Reuters last week the military will have not have an active law enforcement role, instead sending engineers, pilots and other support staff including some that may be able deal with crowd control on the USA side of the border.
The announcement comes as President Donald Trump has been trying to focus on the caravan just a week before the midterm elections.
The US military, at the urging of the White House, plans to send 5,000 troops to the southern border to support the thousands of National Guard personnel and customs officials already there, The Wall Street Journal reported Monday.
"Please go back", he urged them, "you will not be admitted into the United States unless you go through the legal process".
The number of people in the first migrant caravan headed toward the USA has dwindled to about 4,000 from about 7,000 last week, though a second one was gaining steam and marked by violence. Among those illegally crossing the U.S. -Mexico border, McAleenan stressed, are criminals and "hardened smugglers".
The new group, whose members called themselves a second caravan, gathered on the global bridge leading from Tecun Uman to Mexico.
But Mexican officials seem intent only on seeing the caravan melt away as it travels toward the U.S. border. The caravan arrived in sufficient numbers to eventually overwhelm the border security forces around the bridge and make it into Mexico. When this group tried to cross into Mexico through the river on Monday but were stopped again by Mexican federal police who insisted they register and show travel documents before entering the country. On Monday he tweeted: "This is an invasion of our Country and our Military is waiting for you!" O'Shaughnessy, Commander of the North American Aerospace Defense Command and U.S. Northern Command provides guidance during a planning and operational update briefing, October 28, 2018.
"We are going to hold them, we are going to build tent cities, we are going to build tents all over the place", he said, according to Agence France-Presse (AFP).
R. Gil Kerlikowske, Customs and Border Protection commissioner from 2014 to 2017, said the military can not stop asylum seekers who show up at border crossings to seek protection, and that Border Patrol agents have had no trouble apprehending people who cross illegally.
The US military is prohibited from carrying out civilian law enforcement on US soil unless specifically authorized by Congress.
Trump has been campaigning intensively for weeks, frequently hammering on the migrant caravan issue and stoking anti-immigrant concerns among voters. Others joined it in southern Mexico. One migrant died after being struck by a rubber bullet in that area, Guatemalan firefighters have said.
He added that asylum seekers would remain in detention while their claims were being processed, a move which he said would deter would-be applicants from coming to the US.
But news reports quote unnamed USA officials as saying the White House is weighing a range of administrative and legal actions on grounds of national security to restrict the ability of migrants to seek asylum.
Andy Murray looks to sport management role for future talent
He says that he still wishes to compete in professional tennis but he also wants to expand his prospects and share his knowledge. At the moment he is juggling many balls and he wants to remain focused on what he knows, but over time his company will develop.