US Defence Secretary Jim Mattis has confirmed that two military bases will be used as temporary camps to house migrants who illegally enter the US.
Speaking in Alaska, Secretary Mattis named the Texas bases but did not say whether they would house migrant children or families held together.
The Pentagon said last week it planned to house 20,000 detained children on military bases.
Meanwhile, Mr Trump repeated calls for deportations without judicial process.
Hiring manythousands of judges, and going through a long and complicated legal process, is not the way to go - will always be disfunctional. People must simply be stopped at the Border and told they cannot come into the U.S. illegally. Children brought back to their country......— Donald J. Trump (@realDonaldTrump) June 25, 2018
"People must simply be stopped at the Border and told they cannot come into the US illegally," he said in a tweet on Monday.
Where are the military bases located?
Secretary Mattis told reporters on Monday that Fort Bliss and Goodfellow Air Force Base had been chosen, but added that he "cannot confirm the specifics on how they'll be used".
He also said the military was still working through details, including exactly how much capacity they need at the two bases because "the numbers [of migrants] obviously are dynamic".
Officials from the Department of Health and Human Services visited three bases in Texas last week to examine whether they could be used to house migrant children, US media report.
During his remarks to reporters on his way to Asia to meet with Chinese, Korean and Japanese defence ministers, Mr Mattis confirmed the two bases that had been chosen.
It had earlier been reported by NPR that Ft Bliss outside of El Paso, would be used to house migrant families, and Goodfellow Air Force, outside of San Angelo, would be used for unaccompanied migrant children, but this has yet to be confirmed.
In his remarks, the defence secretary called it a "legitimate governmental function", adding that the military has previously been called upon to house victims of natural disasters, and Vietnamese refugees.
What's the context?
On Monday, the head of US Customs and Border Protection said his agents have temporarily stopped initiating criminal prosecutions of adults who enter the country illegally with children.
In an interview with the Associated Press, Commissioner Kevin McAleenan said it was because of Mr Trump's executive order last week calling for an end to family separations.
The call for more shelter has followed public outcry over detention of migrant children in government facilities.
On Sunday, a 15-year-old boy was reported missing from a migrant children's centre in Brownsville, Texas, police said.
The boy allegedly ran away from the centre on Saturday, but officials did not offer any further details.
The Department of Health and Human Services (HHS) has asked the military to create temporary housing for the thousands of undocumented migrants being detained at the US-Mexico border.
Emails seen by US media revealed the bases would house minors who have crossed into the US without an adult relative as well as those who have been separated from their parents at the border.
The shelters will be run by HHS and not the Pentagon, according to the Associated Press, and facilities may be available as early as July.
US immigration officials say 2,342 children were separated from 2,206 parents from 5 May to 9 June.
While the adults are held in custody pending court appearances, the children are being sent to holding cells, converted warehouses and desert tents under the "zero tolerance" policy introduced in April.
Officials have gone to court to try to lengthen the time children can be held as parents are prosecuted.
What is the president saying?
Following similar comments over the weekend, President Donald Trump tweeted on Monday that hiring judges "is not the way to go" and that "people must simply be stopped at the border".
"If this is done, illegal immigration will be stopped in it's tracks - and at very little, by comparison, cost", he continued. "This is the only real answer - and we must continue to BUILD THE WALL!"
These comments come days after Mr Trump reversed a policy to separate migrant children from their parents amid fierce backlash at home and abroad.
The president has not made a distinction between economic migrants and those seeking asylum in his Twitter posts.
Mr Trump has faced criticism, including from some in his own Republican party, for his choice of language online.
Following Mr Trump's election in 2016, the numbers of migrants held or detained while crossing the border had dropped significantly.
However, since February 2018, the number of migrants crossing the border illegally is up, with arrests last month more than double those in May 2017.
While illegal crossings cannot be accurately counted, border arrests are used as a measure of illegal border crossings.