The White House has vowed to investigate reports that migrant children were forced to remain on board buses overnight as they waited to be re-located away from the border.
Biden press secretary Jen Psaki called it “outrageous” and vowed to probe “how we got to this point, how this possibly happened”.
According to US media, the children used the toilet and ate aboard the bus.
It comes amid an unprecedented influx in migrant children to US facilities.
In March, a record-breaking almost 19,000 unaccompanied migrant minors entered the US through the southern border.
In April the number dropped to around 17,200. More than 50,000 parents and children travelling as families also crossed into the US that month.
Massive drop in children held by border officials
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“There’s no excuse for this kind of treatment,” Ms Psaki said on Friday about the children on buses. “In terms of what the consequences will be, I just can’t predict that before an investigation has concluded.”