Trump administration had unveiled new rules that would allow officials to detain migrant families indefinitely.
The American Civil Liberties Union says immigration authorities in the United States separated more than 1,500 children from their parents at the Mexico border early in the Trump administration, bringing the total number of children separated since July 2017 to more than 5,400.
The ACLU said on Thursday the government told its lawyers that 207 of the 1,556 children separated between July 1, 2017, and June 26, 2018, were under five years of age.
A federal judge in San Diego has given the government until Friday to identify children separated going back to July 2017.
The government had inadequate tracking systems at the time, complicating efforts to locate children.
Volunteers working with the ACLU are searching for some of them and their parents by going door-to-door in Guatemala and Honduras.