Homeland Security Secretary Alejandro Mayorkas vowed that the United States will not expel unaccompanied minors who cross the U.S. southern border.

The number of migrant children in federal custody has surged past 4,000 with roughly 94% of beds for migrant children occupied, sources told Fox News, while adding that migrant children are entering federal custody faster than they are leaving, creating a backlog.

During an interview on "CBS This Morning," Mayorkas was asked when migrants could begin to travel to the U.S., as Biden administration officials, including President Biden, have urged them not to make the trip but have fallen short of calling the situation a "crisis" at the border.

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"In weeks, in several months," Mayorkas said. "We will expand the legal processes that we have already started to rebuild. We already have reinstitute the Central American Minors program that was built in the Obama-Biden administration and that was torn down by the prior administration."

Mayorkas added: "But we well understand that out of desperation, some children might not wait. Some loving parents might send their child to traverse Mexico alone to reach the southern border — our southern border."

"I hope they don't undertake that perilous journey," Mayorkas said. "But if they do, we will not expel that young child. We will care for that young child and unite that child with a responsible parent."

He added: "That is who we are are a nation and we can do it."

Customs and Border Protection releases unaccompanied migrant children numbers monthly.

"DHS has continued our close coordination with HHS as it increases its capacity to care for unaccompanied minors and place them with sponsors. Our goal is to ensure that CBP has the continued capability to quickly and efficiently transfer unaccompanied minors after they are apprehended to HHS custody, as is required by U.S. law, and as is clearly in the best interest of the children," a CBP spokesman told Fox News in a statement.

Customs and Border Protection announced that the agency encountered 100,441 individuals in February, a 28% increase over January, the agency said. Of those, 19,246 individuals were in family units, 9,457 were unaccompanied children (UACs) and 71,598 were single adults.

Mayorkas, who has denied calling the situation a crisis, recently emailed DHS staff urging them to volunteer to help CBP at the border amid what he described as "overwhelming" numbers.

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"Today, I activated the Volunteer Force to support Customs and Border Protection (CBP) as they face a surge in migration along the Southwest Border," Mayorkas said in an email to staff, seen by Fox News.

"You have likely seen the news about the overwhelming numbers of migrants seeking access to this country along the Southwest Border," he said. "President Biden and I are committed to ensuring our Nation has a safe, orderly, and humane immigration system while continuing to balance all of the other critical DHS missions."

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Meanwhile, the president, during an interview on ABC’s "Good Morning America" this week, was asked if the administration needed to clearly tell migrants not to come.

"Yes, I can say quite clearly: don't come," Biden responded, later adding, "Don't leave your town or city or community."

Fox News' Adam Shaw, Evie Fordham and The Associated Press contributed to this report.