Thousands of migrant children have been detained at the U.S. border as the Biden administration admits the situation has become "difficult."

More than 13,000 migrant children are in U.S. custody, according to several news outlets, including CBS and ABC.

Those figures appear to refer to a combination of children being held by both Border Patrol and U.S. Customs and Border Protection.

More than 9,400 children and teens entered U.S. border custody in February, according to data from U.S. Customs and Border Protection. That marks a 60% rise from January.

More than 4,000 migrant children were being held by the Border Patrol custody as of Sunday, including at least 3,000 in custody longer than the 72-hour limit set by a court order, a U.S. official told The Associated Press. The agency took in an additional 561 on Monday, twice the recent average, according to a second official. Both spoke on condition of anonymity to discuss figures not yet publicly released.

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As previously reported by Fox News, Homeland Security Secretary Alejandro Mayorkas characterized the situation as "difficult", but did not call it a crisis.

"The situation at the southwest border is difficult," Mayorkas said in a lengthy statement. "We are working around the clock to manage it and we will continue to do so. That is our job. We are making progress and we are executing on our plan.  It will take time and we will not waver in our commitment to succeed."

A senior Customs and Border Protection official told Fox News on Tuesday the agency is taking in more unaccompanied alien children than it can process.

"I don't see how we keep this many kids when we can't find sponsors," the official, who spoke on condition of anonymity, said.

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The number of migrants being stopped at the U.S.-Mexico border has been rising steadily since last April, and the Biden administration is still rapidly expelling most single adults and families under a public health order issued by President Donald Trump at the start of the COVID-19 pandemic. But it is allowing teens and children to stay, at least temporarily, and they have been coming in ever larger numbers.

The number of migrants attempting to cross the border is at the highest level since March 2019, with Mayorkas warning that it is on pace to hit a 20-year peak for the year.

The Biden administration had previously predicted that as many as 13,000 migrant children could cross the border in May.

Fox News’ Adam Shaw and The Associated Press contributed to this report.