New York City's jail system, under federal scrutiny over abuses, is banning solitary confinement for inmates 21 years old and younger.

Under a rule passed unanimously Tuesday by the city Board of Correction, the practice of punishing such inmates by locking them alone in a cell for up to 23 hours a day would end beginning Jan. 1, 2016, provided the city finds money to increase the number of jailers.

The move expands on a previous de Blasio administration decision to halt the practice -- punitive segregation, or bing time, in jailhouse parlance -- for 16- and 17-year-olds, at the end of 2014. Of 9,811 inmates in city jails, 1,422 are age 21 and younger, according to the Department of Correction.

City Hall is under pressure to curb brutality on Rikers Island, the city's jail complex, which a U.S. Justice Department report last year said suffers from "a deep-seated culture of violence" and makes "excessive and inappropriate" use of "punitive segregation for adolescent inmates."

Dissatisfied with the pace of reform, the federal government disclosed in a court filing last month that it is suing the city.

Among other changes passed Tuesday by the board, which has oversight authority of the department: the maximum punitive segregation sentence will be reduced to 30 days from 90 days per infraction and the maximum time in solitary cannot exceed 60 days within a six-month period, except for the most seriously violent inmates. It also prohibited using solitary for severely mentally ill and physically disabled inmates.

The board also allowed the creation of an "enhanced supervision housing" unit for up to 250 of the worst inmates -- those who are to blame for most violence at Rikers. The city said the unit would be "non-punitive," and feature a higher staff-to-inmate ratio. It would contain services such as a law library, clinics and chaplains so as to reduce the need to move inmates.

In a statement, Mayor Bill de Blasio said the rule change would "lead to a safer and more humane system."

Norman Seabrook, who heads the 9,000-member rank-and-file jailers union, said he worried the rule change would create incentives for inmate misbehavior.

"It think it becomes a cat-and-mouse game of, what this inmate can get away with," he said. "Now he knows that he can do whatever it is that he wants to do."