Los Angeles (AFP) - 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 its feet on reunifying these children with their parents," Lee Gelernt, deputy director of the American Civil Liberties Union's Immigrants' Rights Project, said of the Friday ruling.

- Back to court -

The ACLU brought the case that led to Sabraw's injunction on behalf of migrant parents.

But according to a government official, the judge did not formally rule out extending the period, and another hearing will be held on Monday.

In a filing submitted Thursday, the US Department of Justice said HHS was using DNA swab testing to determine parentage.

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But it said that even though the department "is moving expeditiously to undertake these DNA tests, that process takes meaningful time, even when it is expedited."

It added that given the possibility of false claims, "confirming parentage is critical to ensure that children are returned to their parents, not to potential traffickers," and that the government also needed to determine whether the adults had a criminal history or could present a danger to their children.

The government did not request a specific new set of deadlines, but instead sought to "prepare a proposal for an alternative timeline."

It also sought relief from a paragraph in the original injunction that prohibits the government from detaining adult migrants without their children, arguing it could be read to require the release of such detainees if they had not been reunified within the time frame set by the court.

President Donald Trump, who has made fighting illegal and legal immigration a central plank of his US-centered policy agenda, reversed his government's "zero tolerance" policy on June 20 following public outcry.