The plan is a vast expansion of a new and controversial policy enacted in January where asylum-seekers wait in Mexico while their cases are processed in the US.

The Trump administration plans to send hundreds more Central American immigrants back to Mexico every day while they wait for their asylum cases to be processed, the largest such expansion of a controversial policy that began earlier this year.

Department of Homeland Security Secretary Kirstjen Nielsen ordered US Customs and Border Protection officials to broaden the policy Monday. “The Secretary directed CBP to return hundreds of additional migrants per day above current rates to Mexico,” DHS said in a statement.

It would apply to asylum-seekers who either present themselves at ports of entry or are apprehended trying to enter the US between ports.

As of March 20, more than 300 immigrants from Central America have been taken back to Mexico as they wait to have their asylum claims heard in the US, according to the Mexican government. The policy — known as the Migration Protection Protocols — officially rolled out in January at the port of entry in Tijuana.

“The administration is trying to do everything they can to deter people from coming right now, whether at ports or between ports,” said Theresa Brown, a former CBP adviser who now heads immigration policy at the Bipartisan Policy Center. “They are at a place where they have no control of the flow coming in such numbers and they are desperate to find a solution to reduce the numbers incoming.”

The announcement comes just days after CBP Commissioner Kevin McAleenan said the flow of families crossing the border has pushed the system to a “breaking point” and that his agency had run out of capacity to hold people.

Nielsen also directed CBP to expand the policy from the current locations — in the San Diego region, the Calexico Port of Entry, and the El Paso Port of Entry — to include other regions.

In February, the ACLU filed a lawsuit — on behalf of 11 individuals seeking asylum who were taken back to Mexico — to try to stop the policy from continuing to be enforced.

The Trump administration had previously informed the Mexican government that it was going to be enacting the policy based on a statute stating that certain individuals can be sent back to the contiguous country they arrived from.

In its lawsuit, the ACLU claimed that the statute could not be used against asylum-seekers, and that it violates legal protections that prohibit the removal of individuals to a country where they would face persecution. The group also argued that the administration avoided the regulatory process to institute sweeping changes and did not go through the normally deliberative system that allows for public comment before a new rule is implemented.

A federal judge in San Francisco held a hearing over the suit in March but has yet to rule on the matter and has since requested briefing from both sides over whether the court has jurisdiction over the case.

Nielsen also directed CBP to immediately reassign 750 CBP officers to Border Patrol sectors that are facing an increased level of crossings and directed them to evaluate increasing the number of reassignments.

“The crisis at our border is worsening, and DHS will do everything in its power to end it,” said Nielsen. “We will not stand idly by while Congress fails to act yet again, so all options are on the table. We will immediately redeploy hundreds of CBP personnel to the border to respond to this emergency. We will urgently pursue additional reinforcements from within DHS and the interagency. And we will require those seeking to enter the United States to wait in Mexico until an immigration court has reviewed their claims.”