Buena Ventura Martin-Godinez, center, holds her son Pedro, right, as she is reunited with her daughter Janne, left, at Miami International Airport, July 1, 2018, in Miami, Florida. Martin crossed the border into the United States from Mexico in May w
Buena Ventura Martin-Godinez, center, holds her son Pedro, right, as she is reunited with her daughter Janne, left, at Miami International Airport, July 1, 2018, in Miami, Florida. Martin crossed the border into the United States from Mexico in May w

Hundreds of migrant children, mostly from violence-plagued Central American countries, remain separated from their parents at the U.S.-Mexico border, as American officials face a court-ordered Thursday deadline to reunite them.

U.S. officials and lawyers for the American Civil Liberties Union are appearing again Tuesday before U.S. District Judge Dana Sabraw in San Diego, California, to update him on progress in carrying out his order to return 2,551 children to their parents who were separated weeks ago as they crossed illegally into the United States. They were detained as part of President Donald Trump's "zero tolerance" policy to stop illegal migration into the U.S. at the Mexican border.

After an international outcry, Trump halted the breakup of families, most of them from Guatemala, Honduras and El Salvador. "It's about keeping families together while ensuring we have a powerful border," Trump said at the time.

But Trump's order ending the separation of children from their parents left government officials unprepared to deal with the reunification of families where children had already been split from their parents. In a report to Sabraw on Monday, the U.S. said it has reunited 1,187 children with their families or other relatives.

But U.S. officials said 463 parents of children aged five and older are not in the United States anymore, an indication they may have already been deported without their children. The government said the number is being reviewed but has maintained that those deported willingly left without their children. However, immigrant activists have questioned whether those leaving the United States fully understood what they were agreeing to.

In all, the government says 879 families have been reunited, many of them held at detention centers and shelters around the U.S. while they await their credible fear interviews. 

If they pass the interview, then most will pay bonds and be released into the United States with electronic monitoring devices on the promise to appear at future immigration court hearings. This is standard legal proceeding for asylum claims for those who did not enter the country at a port of entry. 

If an immigrant entered the United States through a port of entry, they are then classified as "arriving aliens" and are not bondable by a judge- only Immigration and Customs Enforcement can set a bond or parole them. 

Another 538 parents are in government custody and cleared for reunification with their children but are awaiting transportation.

Of the 2,551 separated children, 1,634 have been cleared for potential reunification, with nearly 200 ineligible for one reason or another. About 260 cases are under review. More than 460 are believed to have left the U.S.

Of the parents still in the U.S., about 900 are facing deportation. But Sabraw has temporarily blocked sending them back to their homelands as he weighs whether to give them time to consider their options once they have been reunited with their children.

Immigration Reporter Aline Barros contributed to this report.