711 migrant children won't be reunited with parents by deadline: Official
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WATCH: More than 1,000 parents have been reunited with the children but more than 900 were deemed ineligible.

All eligible migrant children who have been separated from their parents at the U.S. border will be reunited with their parents by the end of the day, a Department of Homeland Security official told ABC News.

However, there are still 711 children who are not being reunited with parents, guardians or sponsors.

By Thursday evening, 1,820 children had been discharged from U.S. Department of Health & Human Services custody, the official said. Of those children, 1,442 were reunited with their parents in U.S. Immigration and Customs Enforcement custody, and 378 children were released to other guardians or sponsors in the U.S., the official said.

This U.S. Customs and Border Protection photo obtained on June 18, 2018, shows intake of illegal border crossers by U.S. Border Patrol agents at the Central Processing Center in McAllen, Texas, May 23, 2018.

The Trump administration is facing a court-ordered deadline to reunite more than 2,500 immigrant children by day's end. A total of 1,637 parents were ruled "eligible" for reunification at a status conference in San Diego before District Judge Dana Sabraw on Tuesday.

About 463 parents of children still in government custody are no longer in the United States, according to government attorneys. In addition, parents have not been identified for 37 children, the government said, adding that some of those children may be unaccompanied minors who were classified incorrectly.

A two-year-old Honduran girl cries as her mother is searched and detained near the U.S.-Mexico border on June 12, 2018, in McAllen, Texas.

Most of the reunifications are taking place in Karnes, Texas, according to the government.

Earlier this year, the Trump administration enacted a "zero-tolerance" immigration policy on people entering the U.S. illegally.

Central American asylum seekers wait as U.S. Border Patrol agents take them into custody on June 12, 2018, near McAllen, Texas.

The children who were reunited with their families were ages 5 and over, according to a Department of Justice court filing obtained by ABC News.

Many of the children have not seen their parents in weeks or months, according to the AP.

The government and the ACLU are scheduled to be back in court in San Diego on Friday.

ABC News' Lauren Pearle contributed to this report.