A California judge on Friday gave the US government until the following night to submit a list of children under five separated from their families at the border, a government official said.
US President Donald Trump's administration had asked the court to extend a deadline to reunite some detained children with their migrant parents, arguing it needed more time to perform necessary checks and confirm identities.
Judge Dana Sabraw had issued an injunction on June 26 requiring the government to reunite detained migrant children under the age of five within 14 days and those over that age within 30 days.
More than 2,300 children, around 100 of them under the age of five, were separated from their families as a consequence of the Trump administration's "zero tolerance" policy that saw their parents prosecuted for illegally crossing the border, even if they did so to seek asylum.
Several hundred have already been reunited with their parents, but the government has struggled to keep up, and it admitted Thursday to using DNA tests to determine parentage.
In total, about 11,800 migrant children are currently detained by US authorities after having crossed the border illegally, but 80 percent of them are teenagers who arrived alone, Health and Human Services (HHS) Secretary Alex Azar has said.
"The judge made it very clear he wasn't going to allow the Trump administration to drag it...