General of the USA air force on meet the press said that "to the end of the week referred to the military contingent will be placed in the South-Western sector of our border, and this is only the beginning of large-scale operations". The plan, known as Operation Faithful Patriot, will see 1,800 troops sent to Texas, 1,700 to Arizona, and 1,500 to California.
The mobilization will include three combat engineer battalions, members of the U.S. Army Corps of Engineers and troops who specialize in aviation, medical treatment and logistics, O'Shaughnessy said.
"We'll be able to spot and identify groups and rapidly deploy CBP personnel where they are needed", O'Shaughnessy said.
National Guard troops routinely perform those same functions, so it remains unclear clear why active duty forces would be used.
USA officials last week said about 800 active-duty troops would be sent to provide assistance - mainly in the form of logistical support - to border guards working along the frontier.
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. A second group of about 3,000 that clashed with Mexican police over the weekend lags behind them.
US officials have stressed that the troops would not police the border and instead carry out support roles like building tents and barricades, and flying USA customs personnel to locations along the border.
"Right now, we're planning it, how we're going to do it", he added.
Immigrant advocacy groups and the American Civil Liberties Union blasted the move to send military forces and said migrants are exercising their rights under global and federal laws to seek asylum in the United States.
Some of the migrants and refugees hope to eventually enter the United States to escape violence and poverty.
Advanced helicopters will enable border protection agents to swoop down on migrants, he said.
The US military operation drew quick criticism.
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. However, almost all of the kinds of troops sought for Faithful Patriot exist in the Guard.
The troops will come from Army bases around the country, including Fort Bragg in North Carolina, Fort Riley in Kansas, Fort Campbell in Kentucky and Joint Base Lewis McChord in Washington state.
The announcement comes just days after President Trump declared the migrant caravan making its way to the USA a "National Emergency".
Military personnel are legally prohibited from engaging in immigration enforcement.
Many basic questions remain unanswered a day after the Pentagon announced an open-ended deployment of over 5,200 active-duty troops to the border, including the cost and scope of the mission as well as the Pentagon's assessment of any threat posed by arriving migrants.
Military police units are also being deployed. Combined command posts will be established to integrate US military and CBP efforts. The military troops are meant to assist the border patrol, not engage directly with migrants, several officials said.
The president has stepped up his focus on immigration in the days leading up to the midterm elections.
Trump has been campaigning intensively for weeks, frequently hammering on the migrant caravan issue and stoking anti-immigrant concerns among voters.
He continued his threats on Monday, tweeting, without providing evidence, that, "Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border".
"Please go back", the president tweeted Monday.