The announcement, just days before the midterm elections, came as Mexico also cracked down on migrants attempting to cross its own porous southern border.
Good news is, the "caravan" is still 1,000 miles away from the nearest crossing at McAllen, Texas, AP reported, and the trek "could be twice as long if the group of some 4,000 migrants heads for the Tijuana-San Diego frontier, as another caravan did earlier this year".
The troops are expected to perform a wide variety of functions such as building tents and barricades and transporting supplies for the Border Patrol, and not be involved in policing the border, officials said.
Gen. Terrence O'Shaughnessy tells reporters that 800 soldiers were en route on Monday and that the remainder of the troops would be at the southwest border by the end of the week.
At a meeting brokered by Mexico's National Human Rights Commission, police said they would reopen the highway and only wanted an opportunity for federal authorities to explain the proposal to migrants who had rejected it the previous evening. 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.
26, that migrants wishing to obtain temporary identification documents, jobs, and/or education for their children could do so by registering for asylum in southern Mexico.
The move represents a massive military build-up along the border, where some 2,000 National Guardsmen are already working to provide help to overwhelmed authorities.
But Mexican officials seem intent only on seeing the caravan melt away as it travels toward the United States border.
The massive deployment - about five times more troops than Pentagon officials indicated they initially planned to send last week - would bring to the border a military force roughly equal to the USA military deployment in Iraq and Syria to fight the Islamic State.
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Speaking to Fox News, Mr Trump said migrants would be housed in tents, rather than released, pending court decisions. One of the migrants died from the confrontation. About 600 migrants in the second group tried to cross a bridge from Guatemala to Mexico en masse Monday. "We want to work with Mexico to manage that flow".
"If you're already seeking asylum, you've been given a generous offer", he said of Mexico.
But news reports quote unnamed US 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.
Thousands of U.S. troops may be headed to the border with Mexico ahead of a caravan of migrants trekking toward the United States, U.S. officials told Reuters on Monday, offering much higher estimates than the 800 to 1,000 initially forecast.
His experience with the caravan had taught him one thing, he said: "The next time, I'm going to need a 'coyote, '" or smuggler. They will not have arresting power and won't interact with migrants.
"If you wanna wait, they don't usually get asylum".
Trump also said the group includes "Gang Members and some very bad people".
United States officials have told Reuters the support mission would be coordinated by the U.S. military's Northern Command.
The U.S. military has already begun delivering jersey barriers to the southern border in conjunction with the deployment plans.