United States President Donald Trump has backflipped on a policy that he denied he had, blamed on the Democrats and claimed that only Congress could fix.

Yet, with the stroke of a pen the President has stopped the separation of migrant children and parents who've crossed the border into America.

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What happens to the roughly 2,000 children who are already in detention or foster care?

That's unclear.

Here in McAllen, Texas, near the Mexican border, large groups of migrants who'd crossed from Honduras, Guatemala and El Salvador were still being released into the US this week under the old so called "catch and release" policy.

Caught after crossing outside an official border post, detained for a few days, allocated a court date, and then released, they were the lucky ones.

One man who was separated from his teenage son for two days before they were both released observed other families who were split.

"There was one boy five years old; another girl was one," he said through a translator.

"They wanted to separate her. Her father was crying a lot because they wanted to take his girl from him, it was very hard."

Many migrants are fleeing violence in their home countries, Sister Normal Pimentel says. ( Supplied )

Catholic Charities of the Rio Grande Valley has been running a transit centre for incoming migrants since the trickle became a flood in 2014.

Sister Normal Pimentel says most are fleeing gang and domestic violence and crime in their home countries.

"Generally, when you see them walk through our doors, their faces reflect a lot of anguish and stress," she said.

"They've been through so much you can see in their eyes how much they've suffered along the way. The journey has not been easy.

"The very first question I ask is 'como estas'? How are you? And, sometimes in that moment, it's almost like you capture that extreme, profound pain and how much they've been through."

However, Mr Trump says that tougher policies are needed to prevent criminals from violent gangs like MS-13 entering the United States.

Separating parents from children is something the administration chose to do under existing law to deter people from using children to get an easier ticket into the country.

Lawyer Carlos Garcia, who has been at court in McAllen helping parents whose children have been taken away, rejects that.

"I think that argument is disgusting because that's not the way we should treat other human beings," Mr Garcia said.

"As an American, that makes me so upset because that's not what I know our country is about, and it makes me emotional just thinking about that, because I think about the parents that I've talked to. And in general, I think about parents and their children and it's so sad."

Sorry, this video has expired US Border Patrol video shows immigrant children locked in cages at Texas holding facility

While today's executive order officially ends family separations, it leaves room for families to be split if a justifiable reason can be found.

The overall "zero tolerance" approach to people crossing the border outside official entry points will also continue.

That means that adults will be prosecuted via the criminal justice system and while families are together they could be held long term, but only with changes to existing law.

Previous policy under Barack Obama that resulted in families being held for long periods attracted a barrage of legal challenges.

People held signs of protest against Mr Trump's previous executive order. ( Reuters: Patrick T Fallon )

Outside the federal court in McAllen, buses have been pulling up in the early mornings filled with people being prosecuted under the zero tolerance approach.

That's likely to continue.

Meanwhile, Mr Garcia remains concerned about the families that have already been split up with children placed in detention or dispersed to foster care in distant states.

The administration is not guaranteeing reunification.

"I don't think anything is going to happen to them," Mr Garcia said.

"Their child is going to remain in immigration custody and that child is going to confront an immigration judge by him or herself.

"A child — a four or five-year-old — without any representation.

"And then that adult person is also going to have to figure out and manoeuvre their way through the immigration court process."

Sorry, this video has expired Heartbreaking audio of separated migrant children played outside Trump Hotel fundraiser

But Mr Trump's hardline immigration policy, in part, got him elected.

And he has plenty of support in McAllen.

Conservative radio host Sergio Sanchez is one cheerleader.

"As I was explaining on my talk show, the majority of states in this country voted to bring in an outsider, Donald Trump, to try to fix this mess," he said.

"There are right ways to do things and there are wrong ways to do things and jumping the line is wrong and as I passionately express, I never hear, ever hear, national media point the finger of blame back at Mexico City or the incompetent, corrupt governments back in Central America who are not taking care of these people, who are not providing proper protection and opportunity for them.

"That's a big story that needs to be told."

Border agent spokesman Chris Cabrera also supports adherence to the law.

"There's a very simple solution to stop separating these families: stop coming across the river illegally," Mr Cabrera said.

"You can still come through the port of entry. You can claim asylum there and not break the law, but once you cross that river, whether it's undocumented immigrant or a United States citizen or whatever, if you cross in an area that's not designated as a point of entry, you break that law and you're subject to arrest, just like any other law."

It's unclear what will happen to the children already in US custody. ( Supplied )

At the city bus station in McAllen, volunteer Luis Guerrera has spent five years shepherding migrants onto buses to other parts of the US.

This week was no different.

"I show them how to go their destinies. What bus to take," he said.

Those who've slipped through Mr Trump's immigration net are still boarding to unknown destinations.

For those adults and children in detention, it's a case of destiny unknown.