Seventeen states have sued the Trump administration to force thousands of immigrant children to be reunited with their parents.
New York and California are among the states which have launched legal action against the president as pressure mounts on his administration to reconnect families separated at the US border more quickly.
They joined forces as more than 2,000 children have been separated from their parents in recent weeks and placed in government-contracted shelters.
Donald Trump was pressurised into abandoning the policy of splitting up children from their parents caught illegally crossing the US-Mexico border.
However, few families have been reunited since he issued an executive order last week to instead detain families together.
“The administration’s practice of separating families is cruel, plain and simple,” New Jersey attorney general Gurbir Grewal said.
“Every day, it seems like the administration is issuing new, contradictory policies and relying on new, contradictory justifications.
“But we can’t forget: The lives of real people hang in the balance.”
The 17 states are all led by Democratic attorney generals and joined Washington DC to file the lawsuit in federal court in Seattle.
In Los Angeles, immigration rights activists separately asked a federal judge to order the release of parents so they can be immediately reunited with their children.
The Justice Department did not immediately respond to a request for a comment on the lawsuit and said it had no comment on the Los Angeles filing.
Health and Human Services Secretary Alex Azar said his department still has 2,047 immigrant children in custody, separated from their parents.
The Trump administration disclosed almost no information on how families would be reunited or how long it would take.
Officials have been looking for spaces to detain migrants, with the Pentagon planning to hold up to 20,000 at US military bases.
The government has also asked the courts to allow authorities to detain families together for an extended period while their cases are resolved.
Under a 1997 law, children must be released from detention as quickly as possible.
Dozens of people in several states, including Florida and Texas, have been protesting.
On Tuesday, 25 demonstrators in Los Angeles were arrested at a rally ahead of Attorney General Jeff Sessions’ appearance at the conservative Criminal Justice Legal Foundation at a hotel on Wednesday.
He was told to “stop caging families” as clergy members blocked the street with a human chain before being handcuffed by police and led away.
Vice President Mike Pence maintained the anti-immigration stance during his first stop of a Latin American tour on Tuesday.
Speaking in Brazil, he warned Central Americans thinking of emigrating to the US that if they “can’t come legally, don’t come at all”.
He will meet the presidents of Guatemala and Honduras and the vice president of El Salvador on Thursday in Guatemala to discuss US immigration.