Prison staff across the United States use unnecessary, excessive and malicious force on inmates with mental illnesses, Human Rights Watch alleged in a report released Tuesday.

The 127-page report, based on interviews with more than 125 people in the mental health and criminal justice system, details incidents where mentally ill prisoners were abused, and offers solutions for how to improve the system. Titled, "Callous and Cruel: Use of Force Against Inmates With Mental Disabilities in US Jails and Prisons," the report mainly focuses on prisoners suffering from illnesses such as bipolar disorder, schizophrenia and depression.

Correctional staff sometimes sprayed mentally ill prisoners with painful chemicals, shocked them with powerful electric stun weapons and strapped them to restraining chairs or beds for days at a time. "Staff have broken prisoners' jaws, noses, ribs; left them with lacerations requiring stitches, second-degree burns, deep bruises, and damaged internal organs," which in some cases "led to their death," the report says.

"Custody staff are not trained in how to work with prisoners with mental disabilities, how to defuse volatile situations, or how to talk prisoners into complying with orders," said Jamie Fellner, report author and U.S. program senior adviser at Human Rights Watch. "All too often, force is what staff members know and what they use. In badly run facilities officers control inmates, including those with mental illness, through punitive violence."

The report notes how imprisonment can act as a catalyst for mental illness, or exacerbate already-present symptoms, and blames the overwhelmed mental health care system in the U.S. for compounding the problem.

With an estimated one in five prisoners in the U.S. suffering from a serious mental illness, and an estimated 5 percent actively suffering from a psychotic episode at any given moment, it's not hard to see why the man in charge of the nation's largest jail, Illinois' Cook County jail, says he actually runs the largest mental health institution in the U.S.

In 2012, an estimated 4.1 percent of adults in the U.S. suffered from a serious mental illness, according to the National Institute of Mental Health. Compare that to the 36.6 percent of prison inmates and 43.7 percent of jail inmates who have been diagnosed with a mental health disorder, per a 2012 Justice Department report.

Human Rights Watch recommends increasing community health resources and ending solitary confinement for mentally ill prisoners, among other solutions.

"Public officials should reduce the number of prisoners with mental disabilities confined in prisons and jails, including by increasing the availability of community mental health resources and access to criminal justice diversion programs," the group said. "Officials should also improve mental health services in prisons and jails; and ensure that correctional facilities adopt and follow sound policies on using force that take into account the unique needs and vulnerabilities of prisoners with mental illness. Officials should use training to enforce these policies and include mechanisms for holding accountable staff who violate them."

The United Nations Human Rights Council also criticized the U.S. over its human rights records on Monday in Geneva, reprimanding it over police brutality, detention of prisoners at Guantanamo Bay prison in Cuba, racial injustice and its continued use of the death penalty, the Associated Press reported.