On Friday, Trump tried to walk back those comments, telling reporters that "if our soldiers", or Border Patrol or Immigration and Customs Enforcement officers "are going to be hit in the face with rocks, we're going to arrest those people".
The disclosure by USA officials, who spoke to Reuters on condition of anonymity, illustrates the tension within the administration over using military resources to fortify the border against illegal immigration, a top election issue for President Donald Trump's base.
He has said that the total number of troops deployed to the southern border could ultimately be as high as 15,000. They said they won't be going out on patrol. The officials said that while there had been informal talks between the Department of Defense and DHS about having the military build tent facilities to house detained migrants, tents were never part of the formal request for assistance submitted by DHS and approved by Secretary of Defense Jim Mattis.
Trump then said in a speech at the White House last week that he plans to build "massive cities of tents" to detain migrants, who would be held in those facilities indefinitely while the authorities weighs their asylum request. There are reports that this deployment could cost upwards of $200 million.
Manning said more than 7,000 active duty troops were expected to "soon" be supporting the Department of Homeland Security.
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The troops heading for the border areas of Texas, Arizona and California are a small fraction of the nation's roughly 1.3 million active-duty service members. It was unclear how many had taken up missions on the border, which will include support tasks like building housing for Customs and Border Protection personnel, and erecting barriers.
In 2010, Obama ordered the deployment of up to 1,200 National Guard to the US-Mexico border as part of Operation Phalanx.
The President confirmed last week that his administration was prepared to send upwards of 15,000 U.S. troops to the region to help prevent the "migrant caravan" from entering the United States illegally. The administration says it needs to harden border security as the Central American caravan heads north.
Trump's push to send the military to the border comes ahead of Tuesday's mid-term congressional elections and has triggered sharp reactions, with critics calling it a political stunt that misuses USA military resources. Mr. Trump has blasted illegal immigration in the past, and called for the importance of border security.