Mr Trump signed an executive order requiring that immigrant families be detained together when they are caught entering the country illegally for as long as their criminal proceedings take.

But that may violate a court settlement on how long children may be held, setting up a potential legal fight, unless Congress acts on the issue.

"It's about keeping families together while at the same time making sure that we have a very powerful, very strong border," Mr Trump said as he signed the order in a hastily arranged Oval Office gathering minutes before departing for a campaign event.

The Trump order, an unusual reversal by him, also moves parents with children to the front of the line for immigration proceedings and asks the Department of Defence to help house them.

But it does not end a 10-week-old "zero tolerance" policy that calls for criminal prosecution of immigrants crossing the border illegally.

First Lady Melania Trump and Mr Trump's daughter Ivanka were among those who had urged the President to do something.

"I think anybody with a heart would feel very strongly about it."

The move was the most significant policy reversal by Mr Trump since he took office in January 2017.

It creates a series of new headaches for the administration, as it wrestles with where to house families that are detained together, possibly for long periods, and how to reunite families that already have been separated.