The Department of Homeland Security announced the change in a statement with acting DHS Secretary Kevin McAleenan saying: "This rule allows the federal government to enforce immigration laws as passed by Congress."
The rule ending the Flores Settlement Agreement, which has been in place since 1997, will be published in the Federal Register on Friday and will enter into force 60 days later, if approved by the courts, the Efe news reported.
The Flores agreement establishes that undocumented minors being held in US custody must be provided with housing, medical attention, education, nutrition and proper hygiene, and judicial rulings have prohibited the US government from holding families with children for more than 20 days.
McAleenan said that the new rule is designed to target a 2015 "reinterpretation of the Flores Settlement Agreement" in which a California district court ruled that accompanied minors are subject to the same detention limits as unaccompanied minors.
That 2015 ruling "has generally forced the government to release families into the country after just 20 days, incentivizing illegal entry, adding to the growing backlog in immigration proceedings, and often delaying immigration proceedings for many years," said McAleenan.
McAleenan said on Twitter in a series of posts that the government's so-called Flores Final Rule "allows DHS to keep families together during fair and expeditious immigration proceedings, and eliminates a key incentive that encourages traffickers to exploit children."
He added that "Today's action addresses a court-imposed weakness in immigration law that prevented DHS from detaining a family together for more than 20 days and codifies critical commitments on the conditions for children in Federal care."
Although it has taken steps to halt the migrant flow, the Trump administration has increased its efforts since in May a record of 1,32,870 undocumented migrants were detained on the border with Mexico, of whom 84,491 were in family groups, which authorities define as individuals accompanied by a minor, a parent or a legal guardian.
US immigration authorities say that they have found cases of minors being used by unrelated adults to illegally enter the country, knowing that the authorities must release their "family group" within 20 days.
DHS said on Wednesday that the Flores agreement originally was to have remained in force for no more than five years but it has been extended without previous administrations adopting a definitive rule.
The department also said that the Trump administration will continue to work toward establishing a more effective immigration system.
The move, however, was criticized by many pro-immigrant and Democratic organisations, including FWD.us, a group defending immigrants' rights cofounded by Mark Zuckerberg, Bill Gates and other tech business leaders.
Meanwhile, the head of the Democratic National Committee, Tom Perez, called the administration's measure "immoral (and) disgraceful" and said that it shows the "bottomless cruelty of this administration."
"Asylum seekers and immigrants deserve to be treated with basic human dignity and decency, but once again, Donald Trump and his Republican allies are demonising them to divide the American people. There is no moral justification for the indefinite detention of children and no excuse for the trauma this policy will inflict on families," Perez said.
And the President of the Families Belong Together coalition, Jess Morales Rocketto, said in a statement that "This rule will separate more families and traumatize countless others in the process."
( With inputs from IANS )