More than 3,600 migrant children were being detained in US border centres as of Thursday morning, a US official said.
That was more than four times the number in late February, a sign of a growing humanitarian and political crisis for President Joe Biden's administration.
The number of mostly Central American unaccompanied minors arriving at the border has risen rapidly in recent weeks.
That means more children stuck in border patrol stations while they await transfer to increasingly crowded federal shelters and eventual release to parents or other sponsors.
The border stations were built to house men for short periods and could pose a Covid-19 health risk to children and staff if they become too crowded.
Last week, US health officials lifted coronavirus-related capacity limits on shelters for unaccompanied minors to ease the housing problem, but beds have been filling up quickly.
Mr Biden pledged to take back many of the restrictive policies of former president Donald Trump.
In February, he started allowing unaccompanied minors arriving at the border to enter the country.
They had previously been sent back to Mexico or quickly deported under a Trump order known as Title 42.
Republicans have condemned Mr Biden, saying his administration has encouraged illegal immigration.
At the same time, some Democrats have criticised Mr Biden for keeping some Trump policies and for reopening an emergency shelter in Texas that was used under his predecessor.
The arrivals resemble previous surges of unaccompanied minors and families in 2014 and 2019, officials and experts say.
The 3,600 children in US Customs and Border Protection custody is up from about 800 on February 22.
The agency did not immediately respond to a request for comment.
