The Trump administration may have deported more than 450 migrant parents without their children after separating them at the U.S. southern border, according to a new court filing.

Administration officials on Monday said 463 parents of separated children aged 5 years and older have potentially been deported without their children. Their files are under review, the government’s status update said, and that number could change.

The immigrant families were separated as a result of the Trump administration's "zero tolerance" immigration policy, which called for prosecuting all adults who illegally cross the border.

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Government attorneys and administration officials have maintained that parents who were deported without their children willingly gave up custody.

But immigration advocates have expressed concern that parents who were deported without their children signed papers waiving their rights without knowing what they were signing.

Attorneys for the American Civil Liberties Union (ACLU), which is bringing a lawsuit against the Trump administration for the separations, said in the court filing they have yet to receive a full list of parents who have been removed from the country.

ACLU attorneys also said the administration has not yet provided a full list of parents with removal orders who have signed forms electing to be deported without their children.

“These parents urgently need consultation with lawyers, so they do not mistakenly strand their children in the United States,” the ACLU said.

A federal judge has ordered the government to reunite all eligible parents with their children by Thursday. The Trump administration has identified 1,634 parents eligible for reunification with their children, out of a possible 2,551 children in custody.

Of the parents still in the country, 900 are already facing a final order of deportation, the administration said.

U.S. District Judge Dana Sabraw has temporarily ordered a pause on all deportations of reunited families in order to consider a request by the ACLU to give parents time to fully consider the decision whether to have their children deported along with them.