US officials likely lost track of nearly 6,000 unaccompanied minors last year – a substantial increase from the 1,500 children the Trump administration admitted they could not locate earlier this year.

The Department of Health and Human Services (HHS) came under fire in April after officials said they were unable to reach some 1,475 immigrant children they had placed with sponsors inside the US. But based on the administration's own numbers, the department likely lost track of thousands more children over the last fiscal year.

Under current US law, children who arrive at the border without parents or guardians are transferred to HHS custody. The department then attempts to place these children with family members, friends, or other sponsors inside the US while their asylum claims are processed.

HHS recently called the sponsors of more than 7,000 children they had resettled between October 2017 and the end of the year. Of the 7,635 children the department tried to locate, only 6,075 remained with their sponsors. Twenty-eight had run away, five had been removed from the country, and 52 had relocated to live with a non sponsor.

A total of 1,475 children could not be reached at all – sparking headlines about the migrant children “lost” by the Trump administration.

Screaming children heard crying for parents at US detention centre after being separated at border under Trump policy, in distressing audio recording

Stephen Wagner, the acting assistant secretary of the Administration for Children and Families, told reporters that 14 per cent of the 7,635 sponsors never responded to the department's calls. But those sponsors represent only a small fraction of the 42,497 unaccompanied minors the department resettled in the last fiscal year. Using Mr Wagner's 14 per cent estimate, the total number of minors unaccounted for is likely closer to 5,945.

A former HHS employee who placed some of the calls to sponsors confirmed there were “a lot more” children unaccounted for than the government previously indicated.

“You can bet that the numbers are higher. It doesn’t really give you a real picture,” the specialist told McClatchy DC, which first conducted the analysis.

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HHS did not respond to The Independent's for comment. The department previously responded to outcry about the 1,500 “lost” children by noting that HHS is not responsible for the whereabouts of unaccompanied minors once they leave the department’s custody.

HHS officials also objected to the term “lost”, saying there are many benign reasons that children or sponsors do not respond to check-up calls.

“There's no reason to believe anything has happened to the kids,” Mr Wagner said.

The new estimate comes as the Trump administration faces growing criticism for its “zero tolerance” immigration policy, which has resulted in a growing number of children being separated from their families at the border. Experts say the policy will only add to HHS's troubles by increasing the number of children they must place with sponsors.