Migrant children are crammed into a Department of Homeland Security holding facility in Donna, Texas, on March 30, 2021. (DARIO LOPEZ-MILLS / AP)

DONNA, Texas－More than 500 migrant children were packed into plastic-walled rooms built for 32 people, sitting inches apart on mats with foil blankets on Tuesday at the largest US Customs and Border Protection (CBP) holding facility for unaccompanied children.

The CBP's main child processing center, a compound of white tents in Texas' Rio Grande Valley, held over 4,100 migrants. More than 3,400 of them were children who traveled to the US-Mexico border alone. The rest of the migrants being housed were families.

The facility, designed for 250 people under guidelines from the Centers for Disease Control and Prevention (CDC) during the coronavirus pandemic, has had to adapt amid a spike in families and unaccompanied children crossing the border into the US.

In US Customs and Border Protection's largest child processing center, doors to the rooms were open for free movement but there was little room to roam and no one to play games. Most children just sat on the ground close together, chatting quietly. Some were wrapped in foil blankets. Lights are dimmed at night

The US government allowed journalists to see conditions for the first time since the facility opened on Feb 9. It was a grim picture.

A 297-square-meter space had been divided into several rooms for 32 children each under CDC guidelines, each separated by thick plastic walls instead of the chain-link fence used by previous administrations.

Despite the health recommendations, one of the "pods" held nearly 700 kids, another nearly 600 and others had just above 500. Everyone wore masks, but COVID-19 tests are not done unless they show symptoms.

ALSO READ: Homeland Security: US facing biggest migrant surge in 20 years

Doors to the rooms were open for free movement but there was little room to roam and no one to play games. Most children just sat on the ground close together, chatting quietly. Some were wrapped in foil blankets. Lights are dimmed at night.

Separated by age

Children, most of them between 13 and 17, are separated by age. Families occupied a separate pod that was less crowded than the jam-packed rooms for older children.

"I'm a Border Patrol agent. I didn't sign up for this," Oscar Escamilla, acting executive officer of the Border Patrol's Rio Grande Valley sector, said while looking at the younger kids.

Children are processed in the tent facility in the town of Donna before being taken to longer-term care facilities run by US Health and Human Services (HHS) and then placed with a family member, relative, or sponsor.

About two dozen of some 270 children being transferred to HHS midday tested positive for COVID-19, the only time they are tested. Escamilla said the overall positivity rate at the Donna facility was about 14 percent.

More than 2,000 kids have been at the facility for more than 72 hours, including 39 for more than 15 days.

About 250 to 300 children enter the Donna facility daily and far fewer leave, a "lopsided" difference Escamilla said was leading to more crowded conditions. It held as many as 4,600 migrants.

Several hundred kids and teenagers are crossing the border daily, most fleeing violence, poverty or the effects of natural disasters in Central America.

READ MORE: US asylum deals with Honduras, El Salvador, Guatemala halted

A top US border official said on Tuesday that he expects more than 1 million migrants will arrive at the US-Mexico border this year, a sign of a growing humanitarian challenge for US President Joe Biden on the southwest border.

Agencies via Xinhua contributed to this story.