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

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.

It was not immediately clear when Trump's order would be implemented.

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

"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.

The executive order states that immigrant families will be detained together, except in cases where there are concerns about the child's welfare, but it is unclear for how long.

Trump's order also calls for prioritising immigration cases involving detained families.

The President said his wife, Melania, and daughter, Ivanka, who reportedly have been applying pressure on him to drop the policy in recent days, "feel strongly" about ending the practice of separating migrant families.

"I think anybody with a heart would feel very strongly about it," he said. "We don't like to see families separated."

Vice-President Mike Pence and Department of Homeland Security Secretary Kirstjen Nielsen, who has emerged as the face of the White House policy, were both present for Wednesday's order signing.

Republican congressional leader Paul Ryan said the House of Representatives will vote on Thursday "on legislation to keep families together".

He did not immediately provide details of the bill, but said it resolves the issue of so-called Dreamers, undocumented adult migrants who entered the US as children, "in a very elegant way".

On Wednesday, American Airlines, United Airlines and Frontier Airlines said they did not want their planes used by the US government to transport the migrant children.

United chief executive Oscar Munoz said: "We want no part of it."

For days administration officials have insisted they were simply following the law as written and their "zero tolerance" policy for illegal border crossings meant they "have to take the children away", in the president's words.

Critics have countered that Mr Trump unilaterally created the situation that produced the heart-rending accounts of children separated from their parents, and he could unilaterally fix it.

By taking executive action, the president is effectively acknowledging they were correct.

Now the fight will probably move to the courts, with legal challenges to the administration's decision to hold detained families together while their immigration status is adjudicated.

That is more politically hospitable ground for Republicans, who already face challenging mid-term congressional elections. (IANS)