Hundreds of children at a migrant detention center in Texas are being held in “inhumane” conditions that amount to an “emergency public health crisis” and should be allowed immediate access to doctors, according to an attorney who gained rare access to the facility.

Elora Mukherjee, the director of Columbia Law School’s immigrant rights clinic, was one of six attorneys to visit the detention center in Clint as part of ongoing litigation about an agreement that states unaccompanied children can’t be held in US Customs and Border Protection (CBP) facilities for more than 72 hours.

The team found that children had no adequate access to medical care, had no basic sanitation, were exposed to extreme cold and did not have adequate access to drinking water or food.

“I’ve been visiting children detained in federal immigration custody for 12 years,” Mukherjee told the Guardian. “I have never seen anything like this before. I have never seen, smelled, had to bear witness to such degrading and inhumane conditions.”

The UN human rights chief Michelle Bachelet on Monday said she was “appalled” at the conditions. “As a paediatrician, but also as a mother and a former head of state, I am deeply shocked that children are forced to sleep on the floor in overcrowded facilities, without access to adequate healthcare or food, and with poor sanitation conditions,” the statement quoted Bachelet as saying.

Two weeks ago, the attorneys met with 60 children between the ages of five months and 17 years to interview them about the conditions in the facility, which is holding 350 children. Some had bodily fluids including breast milk, urine and mucus stained on their clothes and many were wearing the same clothes they had crossed the border in, days or weeks earlier.

An aerial view of the border patrol facility in Clint, Texas, where attorneys reported migrants had been held in disturbing conditions. Photograph: Mario Tama/Getty Images

In the past, Mukherjee said she would raise concerns about conditions with the lead counsel in the case, who would then pursue a remedy. This time, however, the conditions were so shocking the attorneys were compelled to approach the media.

Mukherjee noted that seven children have died in federal immigration custody or shortly after being released, compared to no such deaths in the 10 previous years. “We were extremely concerned that more children might die if we didn’t go public,” she said.

At Clint, attorneys learned a flu epidemic had left children quarantined, but were blocked from interviewing them in-person to ensure they were receiving proper medical care and instead communicated with some of the oldest children by phone.

A week before their visit to Clint, at a similar facility in McAllen, Texas, lawyers and a pediatrician had identified five detained babies who needed immediate hospitalization and were transferred to a local hospital’s neonatal intensive care unit.

“The conditions within which they are held could be compared to torture facilities,” the physician, Dolly Lucio Sevier, wrote in a medical declaration obtained by ABC News.

CBP initially denied the attorneys’ reports, but its own watchdog, the homeland security department’s Office of Inspector General, had already put together – then released last week – reports warning of dangerous overcrowding in border patrol facilities.

On 10 June, the auditor said it witnessed “serious overcrowding” in four of five facilities and prolonged detention at the other five facilities – of both adults and children – that needed to be addressed immediately.

Overcrowding observed by the Office of Inspector General at the border patrol station in McAllen, Texas, on 11 June. Photograph: Handout/Getty Images

CBP said in a statement it “leverages our limited resources to provide the best care possible to those in our custody, especially children. As DHS and CBP leadership have noted numerous times, our short-term holding facilities were not designed to hold vulnerable populations and we urgently need additional humanitarian funding to manage this crisis.”

In response to the slew of devastating reports, the health department, which takes custody of unaccompanied migrant children until they can be paired with relatives or foster parents, is expanding its shelter network.

And the House oversight committee said it will hold a hearing on Wednesday about the treatment of migrants at detention facilities.

Also next week, an independent mediator in the case that spurred the attorneys’ visit to Clint is due to provide a report about the detention centers before 12 July, according to court documents. That same day, Lights for Liberty vigils are planned around the country to protest the government’s failure to adequately care for these children.

Jennifer Nagda, policy director at the Young Center for Immigrant Children’s Rights, said that the issue stems from the Department of Homeland Security (DHS) treating detention centers as part of enforcement instead of a site for protecting adults and children.

“It is incredibly frustrating when you know that on the part of homeland security, it is not due to a lack of resources, it is due to a lack of intention,” Nagda said. “They have sufficient funds to provide three decent meals a day and a mattress a child can sleep on and a bathroom they can use privately.”

Nagda is one of hundreds, if not thousands, of advocates who have been raising concerns about immigration detention facilities while the US rapidly expanded immigration detention in the past two decades.

The number of detained migrants increased in 1996 after then president Bill Clinton signed a pair of laws that introduced mandatory detentions for asylum seekers and legal immigrants who had committed crimes and allowed for indefinite detention.

Nagda said after a decade working in this field, she was still shocked by the reports that emerged in recent weeks and was concerned a similar situation was replicated at other border facilities.

Despite the grim reality at the border, Nagda clung to the power public outcry could have to change the current conditions. She thinks activists should specifically be pushing for children in detention to have access to pediatricians or medical experts with experience helping children and to have child welfare experts in the facility.

“Those kinds of agency changes will only happen in response to extraordinary public pressure and I think the public should take heart that their anger, and rallying and marches could actually influence how this agency spends money and cares for families arriving at the border,” Nagda said, highlighting the role protests played in bringing an end to family separation in the summer of 2018.