Nine parents who were deported as the administration of US President Donald Trump separated thousands of migrant families landed back into the United States late on Wednesday to reunite with children they had not seen in a year and a half.

The group arrived at Los Angeles International Airport from Guatemala City in a trip arranged under the order of a federal judge who found the US government had unlawfully prevented them from seeking asylum. An asylum advocate confirmed the nine parents were all on board the flight.

Some of the children were at the airport to greet them, including David Xol's nine-year-old son Byron.

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David fell to one knee and tearfully embraced Byron for about three minutes, patting the back of his son's head.

"He was small," David said after rising to his feet. He looked at his lawyer, who accompanied him on the flight, raised his hand about chest-high and said, "He grew a lot."

David, Byron and his lawyer, Ricardo de Anda, then embraced in a three-way hug and exchanged words in their huddle. Byron was all smiles. Father, son, lawyer and family sponsor eagerly left the airport for their hotel.

The reunion was a powerful reminder of the lasting effects of Trump's separation policy, even as attention and outrage has faded amid impeachment proceedings and tensions with Iran. But it also underscored that hundreds, potentially thousands, of other parents and children are still apart nearly two years after the zero-tolerance policy on irregular border crossings took effect.

"They all kind of hit the lottery," said Linda Grimm, lawyer who represents one of the parents returning to the US. "There are so many people out there who have been traumatised by the family separation policy whose pain is not going to be redressed."

Byron Xol waiting for his father at Los Angeles International Airport as they reunite after being separated about one and half years ago during the Trump administration's wide-scale separation of immigrant families [Ringo HW Chiu/AP Photo]

More than 4,000 children are known to have been separated from their parents before and during the official start of zero tolerance in spring 2018. Under the policy, border agents charged parents en masse with illegally crossing the US-Mexico border, then placed their children in government facilities, including some "tender-age shelters" set up for infants.

The US has acknowledged that agents separated families long before they enforced zero tolerance across the entire southern border, its agencies did not properly record separations, and some detention centres were overcrowded and undersupplied, with families denied food, water or medical care.

In June 2018, US District Judge Dana Sabraw ordered the government to stop separating families and reunite parents and children.

At least 470 parents were deported without their children. Some of the kids were held in US government facilities and ultimately placed with sponsors. Others were deported to their home countries.

Accounts emerged of many parents being told to sign paperwork they could not read or understand or being denied a chance to request asylum in ways that violated federal law.

The US Department of Homeland Security referred a request for comment to the Justice Department, which did not respond.

Difficult choice

The American Civil Liberties Union (ACLU), which brought the original family separation lawsuit before Sabraw, asked the judge to order the return of a small group of parents whose children remained in the US. In September, Sabraw required the US to allow 11 parents to come back and denied relief to seven others.

ACLU lawyer Lee Gelernt said Sabraw made clear he would only order the return of people "who were misled or coerced into giving up their asylum rights." That will leave other parents who fled violence, poverty and persecution to decide whether to have their children return to their home countries or remain in the US without them.

"Many are going to make the decision that generations of immigrant parents have made - to leave their child in the US and endure the hardship of separation, but to do it for their child's own safety," Gelernt said.

Xol said that after he and his then-seven-year-old son, Byron crossed the border, they were taken to a US Border Patrol processing centre in South Texas. Xol was charged with illegal entry on May 19, 2018.

Two days later, Xol said an officer told him to sign a document that would allow him and Byron to be deported together. If he did not sign, Byron would be given up for adoption and Xol would be detained for at least two years.

Byron Xol, an immigrant from Guatemala, opening gifts during his birthday party in Buda, Texas. Byron was separated from his father, David Xol-Cholom in May 2018, during the Trump's administration wide-scale separation of immigrant families [David J Phillip/AP Photo]

Xol signed the document, only to have Byron taken away and then get deported to Guatemala. Byron was placed in government facilities for 11 months.

The family's lawyer, Ricardo de Anda, persuaded a federal court to force the US to let a Texas family take in Byron. Since May 2019, Byron has lived with Holly and Matthew Sewell and their two children, with regular video calls to his family.

Holly Sewell brought Byron, now nine, to meet his father at the airport. They planned to go back to Texas to pack and prepare for Byron to move in with his father once Xol is settled in California. Before the reunion, Byron kept asking Sewell, his caretaker, when his father would clear immigration authorities.

"They're almost here, you're doing great," she said. "Count to 1,000."

"999," Byron responded.

She said she was thrilled Byron could see his dad again but sharply criticised the US government's treatment of asylum seekers.

Other families shared similar stories. Esvin Fernando Arredondo was expected to be on the plane. The father from Guatemala was separated from one of his daughters, Andrea Arredondo - then 12 years old and now 13 - after they turned themselves in on May 16, 2018, at a Texas crossing and sought asylum, according to Grimm, his lawyer. He failed an initial screening and agreed to go back to Guatemala.

People want to make this a heartwarming story, but it's not. It's devastating. Holly Sewell

According to Sabraw's ruling, the government deported Arredondo even after the judge had ordered families reunited and subsequently prohibited US officials from removing any parent separated from their child. He's now being given a second chance at asylum under the court order.

Andrea was separated from all family for about a month, living in a shelter as the government struggled to connect children with their parents because they lacked adequate tracking systems. She was finally reunited with her mother, who had turned herself in at the Texas crossing with the other two daughters four days earlier than her husband, on May 12, 2018.

She and her two daughters passed the initial screening interview for asylum, unlike her husband, even though they were fleeing for the same reason. Their son Marco, 17, was shot and killed by suspected gang members in Guatemala City.

Fernando Arredondo from Guatemala reunites with his daughters Andrea, left, Keyli, right, and Alison, second from left, at Los Angeles International Airport after being separated during the Trump administration's wide-scale separation of immigrant families [Ringo HW Chiu/AP Photo]

Arredondo's wife, Cleivi Jerez, 41, arrived at LAX less than an hour before the flight landed with their three daughters in tow, ages 17, 13 and 7.

"Lots of nerves, last night I couldn't sleep," she said in Spanish in an interview after the flight landed.

Jerez said she planned to stay up late catching up with her husband. She planned to rest at their Los Angeles home tomorrow as well, catching up on their 17 months apart before he has to report to an ICE office Friday in San Diego. Alison Arredondo, 7, said she missed going to the park with her father and she wanted to go to one with him in LA.

While the US has stopped the large-scale separations, it has implemented policies to prevent many asylum seekers from entering the country. Under its "Remain in Mexico" policy, more than 50,000 people have been told to wait there for weeks or months for US court dates. The Trump administration also is ramping up deportations of Central Americans to other countries in the region to seek asylum there.

"People want to make this a heartwarming story, but it's not. It's devastating," Sewell said. "There is just no good reason why we had to do this to this child and this family. And he symbolises thousands of others who have been put in this exact same position."