Reports of unaccompanied migrant children being forced to stay overnight in parked buses at the Dallas convention center are "completely unacceptable" if true, U.S. Health and Human Services Secretary Xavier Becerra said Friday.

Dr. Amy Cohen, a psychiatrist and executive director of the advocacy group Every Last One, said a 15-year-old Honduran boy she is working with was held on a bus from Saturday to Wednesday, using the bus bathroom during that time and unable to move about freely or communicate with family. The boy encountered at least three other children who were held as long in the parking lot of the Kay Bailey Hutchison Convention Center, said Cohen, who also has been in contact with another child who was confined earlier to a bus for an extended period. 

It is unclear how many children were kept on buses overnight.

"This is completely unacceptable," Becerra said. "We're quickly investigating this to get to the bottom of what happened, and we'll work to make sure this never happens again. The safety and well-being of the children is our priority."

White House press secretary Jen Psaki said "there's no excuse for this kind of treatment."

The reports are "outrageous, they're unacceptable and they do not meet our standard for child care," Psaki said.

The Honduran boy's experience, first reported by NBC News, comes as Health and Human Services massively expands its capacity to house migrant children until they can be placed with a sponsor in the United States, usually parents or close relatives, while their cases wind through immigration court. It comes in response to the largest influx of unaccompanied children on record.

The department, whose lodging is more suited to longer-term stays than Border Patrol holding facilities, has grown its capacity to about 20,000 beds from less than 1,000 in mid-February. It's opened 14 emergency intake centers, including at the Dallas convention center and other large venues. The Dallas facility opened in February with plans to house up to 3,000 children.

Health and Human Services had 20,397 unaccompanied children in its custody as of Wednesday. 

The government flew the Honduran boy to Seattle to reunite with his mother and uncle after NBC News inquired about his status.

MVM Inc., a transportation contractor for the government, said it has "safely and professionally" transported migrant children and families for more than six years. 

"Over the last seven weeks, the number of children needing escorts in this pandemic environment has increased to more than 7,100, creating challenging travel logistics and resulting in some extended wait times on their way to reunification sites," the company said in a statement. 

MVM said it experienced some delays at a 24-hour regional hub where buses meet to get children on their way to join family, which resulted in "a child staying at that site longer than our target wait time of four hours. This is a violation of our policy and we are conducting an internal review of this incident."

The company said the child had access to an air-conditioned bus, food and snacks, bottled water and personal protective equipment.