The US Department of Justice (DOJ) has filed an appeal to ask a court to reverse its ruling last month that would pave way for the release of detained immigrant mothers and children who entered the US border illegally and held at family detention facilities.

On July 24, US District Judge Dolly Gee of the Central District of California ruled that the Department of Homeland Security (DHS) breached the Flores settlement agreement of 1997, which required juveniles to be released from custody without delay to a parent, legal guardian and adult relative among others.

In its filing, the DOJ said the policies that the court construed as imposing "blanket" detention of female-headed families have been eliminated and the length of detention has been shortened.

It said the family facilities are being transformed into DHS processing centres that can "efficiently process families; conduct health screenings and provide immunisations; preliminarily assess whether family members are eligible to apply for relief or protection to remain in the United States; facilitate access to counsel and legal orientation programs; and release those found eligible to apply for relief or protection within an average of approximately 20 days."

The DOJ warned that if the court order is implemented, the DHS will be required to release all families seeking to illegally enter the US "if their removal takes longer than three to five days to execute" and "even if they possess no legitimate claims for relief or protection from removal."

The court order, the DOJ said, "could undo the progress that has been achieved in reducing the number of families illegally crossing the Southwest border since the summer of 2014."

It said the proposed remedies by the court "could heighten the risk of another surge in illegal migration across our Southwest border by Central American families."

It said DHS' new policies, which were never covered by the court ruling, allow majority of individuals in family facilities to stay there only during the relatively short time needed for essential processing.

Among families booked into a family facility from June 28 to July 11, 2015, it said, more than 60 percent had been released or removed by July 29 or within two to four weeks compared to the last six months in 2014 where only 21 percent were released or removed within 30 days.

According to the filing, in 2014, the number of family members, or children encountered with a parent or legal guardian, apprehended increased by 361 percent to 68,445. This year, the Border Patrol has apprehended 24,901 family members, a 55 percent decrease from the previous year.

With the court ruling, Democrats and immigration activists have revived calls to shut down family detention facilities, according to Politico.

Democratic presidential bet Hillary Clinton has criticised the policy.

Nearly all members of the House Democratic Caucus have signed a letter to end family detention.

"In light of this recent federal court ruling, we urge you to take all necessary and appropriate steps to bring the Department's practices in line with the settlement agreement and the recent court ruling," the House Democrats said.