The Trump administration expects the number of immigrants sent back to Mexico to await their asylum hearings to "grow exponentially" in the next few weeks as it expands the program to additional cities along the southern border.

On a conference call on Friday morning, Department of Homeland Security officials told reporters that the implementation of the new program — officially called Migrant Protection Protocols but more commonly known as "Remain in Mexico" — has been "taking longer than anticipated." Officials declined to comment on how many immigrants have been subjected to the new policy, saying that "the number will grow exponentially."

"A number today wouldn't reflect what the number will be in a week or two," a DHS official said.

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A Mexican government official commented on the program earlier this week, saying that as of February 21, 112 migrants had been sent back, 25 of whom were minors. Seven of those minors were under the age of 12, according to the government report that was shared with the Associated Press and reviewed by CBS News. Everyone that's been subjected to "Remain in Mexico" so far comes from the Northern Triangle countries of Central America: El Salvador, Guatemala, and Honduras.

DHS officials said that the policy didn't apply to unaccompanied minors, "nor will it at any point in the future." Currently, the program is only active in San Ysidro, the port of entry that connects San Diego to Tijuana, the dangerous Mexican border city where shelters are at capacity and cartel violence is rampant.

Officials didn't say which other border towns "Remain in Mexico" would be expanded to, but said "everything is on the table; there isn't any location we've deemed non-operational."

"Remain in Mexico" is one the biggest changes to immigration policy implemented by the Trump administration. When it was announced in December, it immediately drew criticism from immigration advocates who called it a "due process disaster."

A group of immigration advocates — including the American Civil Liberties Union and the Southern Poverty Law Center — officially challenged the policy in February, filing a lawsuit against Homeland Security Secretary Kirstjen Nielsen and her department. The groups claim that the program is in violation of of the Immigration and Nationality Act, the Administrative Procedures Act and various international human rights laws.

On Friday's call, a DHS official referred to the policy as "a tool that Congress has given us." The Migrant Protection Protocols was announced by Secretary Nielsen in December and was not subject to Congressional approval.

Prior to that announcement, asylum seekers had been allowed to live in the United States while wearing an ankle monitor as they waited for immigration court dates.

Under the new plan, some individuals and family units seeking asylum at San Ysidro are processed by immigration officials and then returned to Tijuana to wait for their immigration court date. They receive an 800-number to check the status of their case, as well as a return date for when their asylum claim will be processed. On the call, a DHS official said that access to legal counsel has not been impacted due to the policy.

DHS officials didn't give a specific deadline for the policy's expansion, but said it would be within "the next few weeks."

"I'd like it to happen sooner than later," the official said.