Department of Defense spokesman Chris Mitchell said the Pentagon had received a request from the Department of Homeland Security (DHS) "to construct temporary facilities at six DHS-specified locations to house and care for a minimum of 7,500" migrants.

However, the department "will not provide detention or custodial support for detained aliens at these ICE detention facilities. ICE is responsible for detention or custodial support," the statement said, referring to US Immigration and Customs Enforcement.

Acting Secretary of Defense Patrick Shanahan must sign off on the request, which Mitchell said would happen "very soon."

The defense department also announced on Wednesday that the US Army Corps of Engineers had awarded two companies contracts worth $788 million to build a "barrier wall replacement" and a "vehicle and pedestrian barrier replacement" along the US-Mexico border in California and Arizona, which would be completed early next year.

The announcements come ahead of President Donald Trump's expected Thursday unveiling of a new immigration policy proposal, which The Washington Post reported will focus on "merit-based" migration.

More than 100,000 migrants were arrested after crossing the US-Mexico border in April, according to a US government tally, the second month in a row detentions reached that level.

In total, almost half a million people have been stopped at the border since October 2018.

Overcrowding at border emergency shelters has forced authorities to release some migrants from detention as they await a review of their asylum applications.

The majority of migrants who cross the US-Mexico border without authorization surrender to authorities and file for asylum.

Most come from Central American countries such as Honduras, Guatemala and El Salvador, from which they say they are fleeing due to high levels of violence and poverty.