A federal judge on Thursday ordered an immigrant mother and daughter brought back to the United States after learning during a court hearing that the government had put them on a deportation flight to El Salvador.

The pair were among 12 plaintiffs in a lawsuit brought by the American Civil Liberties Union (ACLU) challenging changes in asylum policies ordered by US Attorney General Jeff Sessions.

Washington DC District Court Judge Emmet Sullivan was hearing a request to stay the plaintiffs' deportations, when the ACLU learned that two of them were already on a flight to Central America, the civil rights group said.

Upon hearing the news, Sullivan ordered their flight returned and suggested that Sessions could be held in contempt of court, according to the ACLU.

An official from the Department of Homeland Security said in an e-mailed statement that the agency is "complying with the court’s order, and upon arrival in El Salvador, the plaintiffs did not disembark and are currently en route back to the United States.”

During the hearing, the judge ordered a temporary stay on deporting the nine women and three children who filed the lawsuit, according to a court filing.

The lawsuit was filed on Tuesday in the District of Columbia by the ACLU. It challenges a recent tightening on standards for seeking US asylum, which makes it far more difficult for those fleeing domestic or gang violence to win the right to remain in the United States.

Sessions has led efforts by the Trump Administration to crack down on illegal immigration, including the adoption of a zero tolerance policy that briefly included separating immigrant parents from their children while they were in US detention.

Meanwhile, the number of immigrant children in US care who have still not been reunited with their families after being separated at the Mexican border has barely budged in the past week, as the government struggles to locate parents no longer in the United States.

Over 500 children out of more than 2,500 separated from their parents by officials at the border remain in the care of the government. The number dipped to 559 this week from 572 last week, according to a court filing on Thursday.

The children were removed from their parents as part of President Donald Trump's 'zero tolerance' policy to discourage illegal immigration. The policy was abandoned on June 20 in the face of criticism at home and abroad.

Shortly after, US Judge Dana Sabraw ordered that the government reunite the families within 30 days.

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