Violence has increased in New York City's jail system over the past seven years even as the city has spent more money to deal with a steadily declining inmate population, the city's comptroller said in an analysis released on Friday.

The annual cost per inmate has increased by more than 42 percent to $96,232 since 2007, the analysis said, while the average daily population has decreased to about 11,400 inmates this year, from nearly 14,000.

In that same time, violent fights and assaults involving inmates or guards have increased. Inmate assaults on staff more than doubled to about 70 assaults per 1,000 inmates this year. Guards using force on inmates has nearly tripled to 370 instances per 1,000 inmates.

The analysis joins a growing list of indictments against the city's jail system, particularly the Rikers Island jail complex, issued by almost every official who has looked into it, including the city's mayor and U.S. Department of Justice prosecutors.

"In an era of declining crime and detention, violence and costs at city jails should be decreasing," Comptroller Scott Stringer, said in a statement on the analysis conducted by his office. "Instead, past leadership at the Department of Correction allowed jail conditions for correction officers and inmates to degenerate."

The amount New York City spends per inmate was more than twice that spent by Miami, Chicago and Los Angeles last year, the analysis said. Much of the rising budget was due to increasing staff overtime costs.

Mayor Bill de Blasio, who took office in January, has promised to reform a jail system he has described as broken.

His administration is currently meeting with federal prosecutors after they produced a report concluding that the city has breached the constitutional rights of teenage inmates by allowing them to be brutalized while at Rikers Island.

Preet Bharara, the U.S. attorney for New York's southern district, said his office will file a civil rights lawsuit against the city if the system does not improve.

The city is also the subject of other lawsuits over various inmate deaths and beatings on the island.

A spokesman for the Department of Correction said he did not have an immediate comment on the analysis.