WASHINGTON • The United States Navy plans to build sprawling detention centres for tens of thousands of immigrants on remote bases in support of President Donald Trump's "zero-tolerance" policy against unlawful migration, a report said.

According to a draft memo obtained by Time magazine, the navy plans to build "temporary and austere" tent cities to house 25,000 migrants across three abandoned airfields in Alabama, 47,000 people at a facility near San Francisco, and another 47,000 at a training centre in southern California.

The document estimates the navy would spend US$233 million (S$317 million) to run a facility for 25,000 over six months, Time said on Friday.

Asked for comment, Pentagon spokesman Jamie Davis said: "The Department of Defence is conducting prudent planning and is looking nationwide at DoD installations should DHS (Department of Homeland Security) ask for assistance in housing adult illegal immigrants.

"At this time, there has been no request from DHS for DoD support to house illegal migrants."

The fate of 2,300 children wrested from their parents at the US border with Mexico remained unclear on Friday, two days after Mr Trump ended migrant family separations.

While the US leader bowed to global outrage over the splitting of families, conflicting messages were contributing to a sense of chaos in the handling of the crisis.

But having been forced into a climbdown, Mr Trump swung back into fighting mode, insisting that he remained committed to the "zero-tolerance" policy that aims to deter the flow of migrants from Central America.

"We must maintain a Strong Southern Border. We cannot allow our Country to be overrun by illegal immigrants as the Democrats tell their phony stories of sadness and grief, hoping it will help them in the elections," he tweeted.

AGENCE FRANCE-PRESSE