U.S. Border Patrol agents are running out of space to hold migrant children who have been separated from their families, NBC News reported Tuesday.

Border agents have been placing children who crossed the U.S.-Mexico border illegally in holding stations. Roughly 550 children are in custody, and of those, nearly 300 have spent more than 72 hours there, NBC reported.

Seventy-two hours marks the time limit for immigrants of any age to be held in the border stations, which are meant to serve as temporary housing facilities.

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Parents traveling with their children are separated from them while the face legal proceedings.

NBC reported that the extended stays are a result of a backlog of children needing to be matched with relatives or foster parents.

Of the 300 children that have been there longer than 72 hours, almost half are under the age of 12, according to the report.

A spokesman for the Department of Homeland Security did not respond to a request for comment from NBC.

The Trump administration recently implemented a “zero tolerance” policy for those who cross the U.S.-Mexico border illegally. The Justice Department and Department of Homeland Security have said they will separate children from their parents in some cases.

President Trump has on multiple occasions blamed Democrats for the policy, despite the call to enforce it coming from his administration.

Chief of staff John Kelly defended the policy last month.

“The children will be taken care of — put into foster care or whatever," Kelly said. "But the big point is they elected to come illegally into the United States and this is a technique that no one hopes will be used extensively or for very long.”