A US family have been removed from a Syrian detention centre for suspected members of Islamic State and returned to the US. The move could have implications for hundreds of children born to non-Syrians who have been in legal limbo since the disintegration of the terror group.

The family of four were taken from al-Roj camp in the country’s north-east over the weekend, sources inside the camp have confirmed to the Guardian. They are believed to be US citizens of Cambodian ancestry and to include children born under Isis rule.

Another American, Hoda Muthana, an alleged Isis propagandist, was left behind. The US secretary of state, Mike Pompeo, claimed there were no legal grounds for her to be repatriated.

Washington has repeatedly urged countries whose citizens were detained in Syria during the collapse of the terror group to repatriate them from two overwhelmed detention centres in which more than 132,000 people are housed. The retort from European states has been that the US should first sort out its own affairs.

While the total number of US citizens in detention remains unclear, the family of four represents a substantial number removed, which could increase pressure on other countries to repatriate their own citizens.

The UK has said a “small number” of children have left Syria in the last 12 months and returned to Britain via third countries, but its officials were not involved in helping them leave Syria. Shamima Begum, who left London aged 15 to join Isis in 2015, was stripped of her UK citizenship in an effort to prevent her fromm returning to the country. She gave birth in a detention camp to a baby boy who was considered a British citizen. He died three weeks later of pneumonia.

Citing privacy concerns, the US state department would not directly address whether the US family had been removed. In response to a query, a spokesperson said: “We are aware of a small number of US citizens present in camps in north-east Syria. American citizens’ services cases in Syria are extremely complicated due to the lack of consular access to the facilities where they are.

“We take seriously all legitimate claims of US citizenship, no matter where those claims are made, and work to verify and handle those claims on a case-by-case basis. The United States will continue to repatriate its citizens, as we have done in the past.”

The collapse of Isis has left a legal minefield for citizens who travelled to join the group, and created headaches for many governments who remain unwilling to send officials to rescue detainees and risk a backlash from the public.

Securing the return of wives and children of Isis fighters has been seen as less complicated and more feasible, and the United Nations has increasingly urged member states to do more, particularly for children.

“The thousands of children of foreign fighters languishing in camps, detention centres or orphanages in Syria, Iraq and elsewhere are among the world’s most vulnerable children,” said Henrietta Fore, the executive director of the UN children’s agency, Unicef. “They live in appalling conditions amid constant threats to their health, safety and wellbeing. They have little family support. While most are stranded with their mothers or other caregivers, many are completely alone.

“In Syria alone, Unicef estimates there are close to 29,000 foreign children, most of them under the age of 12. Some 20,000 children are from Iraq while more than 9,000 are from around 60 other countries. An additional 1,000 children of foreign fighters are believed to be in Iraq.”

Unicef said it had so far helped with 270 child repatriations. “These children [in the camps] are doubly rejected – stigmatised by their communities and shunned by their governments,” said Fore. “They face massive legal, logistical and political challenges in accessing basic services or returning to their countries of origin. All are victims of deeply tragic circumstances and egregious violations of their rights. They must be treated and cared for as children.”

France, Kosovo, Sweden and Germany have recently removed women and children from al-Roj and al-Hawl camps, where Kurdish forces have struggled to contain a spillover of up to 132,000 people from Isis’s former territory. Many of those countries have used the nearby Iraqi-Kurdish border to transfer their citizens out of Syria, defying demands from Damascus that any transfer must be coordinated with its officials.

Some senior diplomats fear detainee cases could be politicised and effectively pawned in return for concessions if their repatriation was managed through the Syrian government.

Others are exploring options with the International Committee of the Red Cross (ICRC), which maintains ties with Damascus. These would probably involve women and children being transferred via the Syrian capital.

North-east Syria has been largely under the control of US-backed Kurds throughout much of the war against Isis and the Syrian government plays no role in administering the camps. It has often claimed that governments operating in the war-ravaged north-east impugn its sovereignty.

Last month Australia’s prime minister, Scott Morrison, said Canberra was exploring options with the ICRC to remove children born to Isis members. About 20 Australians are believed to be in detention in Syria, nearly half of them children. Norway is also understood to be considering this route.