At least nine infants younger than a year old, including one who is just 5 months old, are being held in ICE custody at a rural Texas detention center without care that's legally required.

That's what three immigration advocacy groups claimed in a letter to the Department of Homeland Security's Inspector General and Officer for Civil Rights and Civil Liberties on Thursday afternoon. The groups said there has been "an alarming increase in the number of infants" being held in ICE custody, and urged the department to "intervene immediately" at the Dilley, Texas, facility.

"We have grave concerns about the lack of specialized medical care available in Dilley for this vulnerable population," said the letter from the three groups — the American Immigration Council, the American Immigration Lawyers Association and the Catholic Immigration Network, Inc.

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The advocacy groups alleged the infants have been subject to "lengthy delays in receiving medical attention and lack of appropriate follow-up treatment." They said one infant has been detained for over 20 days.

"ICE is required to meet basic standards of care for minor non-citizens in its custody," the letter said, citing the Supreme Court case Flores v. Reno. "It repeatedly has demonstrated an inability to do so."

The Department of Homeland Security did not immediately respond to an email from CBS News requesting comment on the letter. The department's Office of the Inspector General also not did immediately respond to an email seeking comment.

The infants and their mothers are being held at the South Texas Family Residential Center in Dilley, a rural town with a population of less than 4,000 people. The facility is "located over one hour by car from San Antonio, the nearest major metropolitan center with facilities equipped to provide specialized medical services," according to the letter.

Some mothers at the detention center have reported that their infants lost weight since arriving at the detention center and "are not feeding well due to sudden changes in formula," according to the letter. Other infants have exhibited behavior and sleep challenges during their ICE detainment, according to their mothers.

The complaint also included a letter from Physicians for Human Rights (PHR), a New York-based non-governmental organization.

"PHR expresses grave concern over a reported increase in detained infant children in South Texas Family Residential Center in Dilley, Texas," the group wrote in a letter addressed to DHS Secretary Kirstjen Nielsen. "Given the inherent harms and health risks of child detention, which are exacerbated (sic) in the case of infants and young children, PHR requests that the government exercise its full discretionary authority to secure the release of these families immediately."