WASHINGTON (AP) — The Pentagon will make space available on military bases for as many as 20,000 unaccompanied migrant children detained after illegally crossing the U.S.-Mexico border, a spokesman said Thursday.

The request for temporary shelter — amid a growing political battle over detained migrants — was made by the Department of Health and Human Services and accepted by the Defense Department, said the spokesman, Army Lt. Col. Jamie Davis.

A Pentagon memo to members of Congress, obtained by The Associated Press, said it has been asked to have the facilities available as early as July, through the end of the year. It said HHS personnel or contractors for HHS "will provide all care for the children," including supervision, meals, clothing, medical services, transportation and other daily needs.

It's not clear which bases will be used to house the children. HHS has assessed facilities on four military bases, but the Pentagon said it has not been told which, if any, of the four will be used. The Pentagon said it will have no role in operating the temporary shelters, which would be controlled by HHS.

The four bases already assessed as potential shelter locations are Little Rock Air Force Base in Arkansas, plus three bases in Texas: Dyess Air Force Base, Goodfellow Air Force Base and Fort Bliss.

Defense Secretary Jim Mattis said Wednesday he is not involved in decisions about housing migrant children detained after crossing the border. But he said the Pentagon will provide whatever support is requested by either the Department of Homeland Security or HHS. The Pentagon memo to Congress said that as of Wednesday it had received no request from DHS.

The children who would be housed on military bases are those who cross the border illegally by themselves, as opposed to those accompanied by adults. On Wednesday, President Donald Trump signed an executive order to keep together children and parents apprehended for crossing the border illegally for at least 20 days. The order also directs the Justice Department to fight in court to permanently remove the threat of separation.

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Associated Press writers Matthew Daly and Ricardo Alonso-Zaldivar contributed to this report.