Biden pledged to end solitary confinement. Federal prisons are increasing its use.

Four months after President Joe Biden issued an executive order to overhaul the criminal justice system, promising his administration would ensure federal prisoners are in "safe and humane" confinement and "free from prolonged segregation," the total number of inmates being held in so-called restrictive housing has been climbing, recent data shows.

Figures from the federal Bureau of Prisons analyzed by NBC News revealed that 11,368 inmates were held in restrictive housing — informally known as solitary confinement — as of Tuesday, up 7% from 10,607 inmates on May 28, the same week as when Biden signed his executive order. It's also up more than 11% from the first few months of the Biden administration.

The number has been increasing steadily month over month, with the vast majority of inmates held in special housing units, in which they are segregated from the general population due to safety concerns or as a form of discipline. The federal government houses more than 142,000 inmates in its custody.

"It's very unfortunate that the numbers have only moved the wrong way," said Rep. David Trone, D-Md., who signed onto a letter this week with 11 other Democratic members of Congress urging BOP Director Colette Peters to reduce the frequency and length of solitary confinement.

"We have a real problem," Trone told NBC News.

The numbers come to light as Peters on Thursday faced her first Senate hearing as the bureau's director, replacing Michael Carvajal last month. Carvajal, a Trump administration holdover, stepped down amid criticism of a crisis-filled tenure marked by agency scandals over allegations of unsafe and unsanitary conditions in prisons, staff misconduct and widespread staffing shortages.

At Thursday's hearing of the Senate Committee on the Judiciary, the committee's chairman, Sen. Dick Durbin, D-Ill., said there may be times when solitary confinement is necessary to ensure safety and security in prisons, but asked how the BOP will address the "abuse" of its application.

Peters, who previously led Oregon's prison system for a decade, said the issue is a priority for her.

"This is a complex issue," she said. "This is one I'm still getting up to speed on at the bureau. But our values match in this topic area, senator."

Durbin was among the Democrats who wrote to Peters this week regarding restrictive housing, and suggested its use "for the briefest amount of time, and only in emergency circumstances for the purpose of de-escalation," while providing mental health treatment for those placed into it.

Before taking office, Biden pledged to overhaul the federal prison system, in part "by ending the practice of solitary confinement, with very limited exceptions." In his executive order in May, he directed the Office of the Attorney General to submit a report to ensure that restrictive housing is "used rarely, applied fairly, and subject to reasonable constraints" and that inmates are housed "in the least restrictive setting necessary for their safety and the safety of staff, other prisoners and detainees, and the public."

Attorney General Merrick Garland has until November to provide the report. A BOP spokesperson declined to comment on its status, but told NBC News that leadership is "committed to ensuring that restrictive housing is used and applied fairly" and "working to comply with all aspects of the Executive Order, including its directives related to restrictive housing."

The bureau also said it wants to implement "evidence-based approaches that both decrease restrictive housing entry and speed exit from that setting," and is developing software to better track, analyze and report data of all inmates held in restrictive housing.

"In the cases where restrictive housing is needed, restrictive housing is neither solitary confinement nor isolated," the bureau added.

The United Nations considers solitary confinement as the isolation of a person in a cell for 22 hours or more "without meaningful human contact," and that over 15 consecutive days of it is a form of torture.

Prisoner advocacy groups contend that while facilities may not technically classify certain forms of isolation as solitary confinement, it's the same in practice and can have a negative effect on mental and physical health. Studies show solitary confinement also heightens the risk of self-harm and suicide and may not be effective in combating recidivism.

"It is beyond disappointing that despite President Biden's and Vice President Harris' pledge to end solitary confinement, this administration has doubled down on the use of this torturous practice," said Johnny Perez, the director of the U.S. prisons program at the nonprofit National Religious Campaign Against Torture and a former inmate at New York's Rikers Island jail complex, where he had been placed in solitary confinement as a teenager.

Trone, however, said he's willing to give Peters time to figure out how to reverse the growing number of inmates held in restrictive housing. At Thursday's hearing, she told senators that she should be held accountable first for any problems that continue to persist across the federal prison system.

"She inherited a broken ship," Trone said. "I'm very comfortable with giving her time in getting the house in order."