United States President Donald Trump has surrendered to public pressure and signed an executive order promising to "keep families together" in migrant detentions.

Mr Trump reversed his own policy amid international fury over the separation of undocumented parents and children.

He said he had been swayed by images of children who have been taken from parents while they are jailed and prosecuted for illegal border-crossing.

But the order does not address families already separated by the policy.

US immigration officials say 2,342 children were separated from 2,206 parents between 5 May and 9 June.

"It's about keeping families together," Mr Trump said at the signing ceremony on Wednesday.

"I did not like the sight of families being separated," he said, but added the administration would continue its "zero tolerance policy" of criminally prosecuting anyone who crosses the border illegally.