The Trump administration says it would require extraordinary effort to reunite what may be thousands of migrant children who have been separated from their parents and, even if it could, the children would likely be emotionally harmed.

Jonathan White, who leads the Health and Human Services Department's efforts to reunite migrant children with their parents, said removing children from "sponsor" homes to rejoin their parents "would present grave child welfare concerns." He said the government should focus on reuniting children currently in its custody, not those who have already been released to sponsors.

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"It would destabilize the permanency of their existing home environment, and could be traumatic to the children," White said in a court filing late Friday, citing his years of experience working with unaccompanied migrant children and background as a social worker.

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The administration outlined its position in a court-ordered response to a government watchdog report last month that found many more migrant children may have been split from their families than previously reported. The government didn't adequately track separated children before a federal judge in San Diego ruled in June that children in its custody be reunited with their parents.

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It is unknown how many families were split under a longstanding policy that allows separation under certain circumstances, such as serious criminal charges against a parent, concerns over the health and welfare of a child or medical concerns.

Ann Maxwell, Health and Human Services' assistant inspector general for evaluations, said last month that the number of separated children was certainly larger than the 2,737 listed by the government in court documents. The department's inspector general report didn't have a precise count, but Maxwell said staff estimated it to be in the thousands.

The American Civil Liberties Union, which wants U.S. District Judge Dana Sabraw's order to apply to children who were released to sponsors before his June 26 ruling, criticized the government's position. A hearing is scheduled Feb. 21.