Japan's Justice Ministry disclosed Friday that more than 20 prison guards had repeatedly pushed, slapped and used other forms of physical violence against a group of inmates at a prison in central Japan, raising questions about the extent of prison abuse in the country.

Justice Minister Ken Saito said the assaults at Nagoya Prison were discovered in August when a prison guard spotted an inmate with an eye injury.

An internal probe found that 22 prison officials routinely slapped some inmates in the face, pushed them in the chest, sprayed alcohol in their faces, and threw dishes and other objects at them. The victims included the one with the eye injury.

When correcting the behavior of inmates, prison officials need to consider their human rights, and "actions like these are unforgivable," Saito told reporters. He expressed "deep regret" on behalf of the ministry.

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Saito said the assaults were especially problematic because fatal bullying at the same prison two decades ago had prompted prison reforms. In 2001, a prison guard used a firefighting hose to shoot water at the rear of an inmate, causing rectal rupture and an infection resulting in death. A year later, five prison guards were charged with assault over their use of a restraining device that resulted in the death of two inmates and the injury of a third.

While the internal probe continues at the prison, Saito said he ordered an investigation by outside experts, as well as a survey of prisons across the country.

The Tokyo Detention House

An investigation has started regarding the repeated assaults at a prison in central Japan by inmates. The Nagoya Prison assaults promoted a country-wide investigation. Pictured is the Tokyo Detention House.  (Kiyoshi Ota/Bloomberg via Getty Images)

The prison officials were in their 20s and 30s, and most had less than three years of experience, he said.

Japanese prisons are known for strict discipline and a lack of access to outside exercise or medical care. Japan also faces criticism over its lack of transparency in carrying out capital punishment.

Saito said the ministry will step up human rights education for prison officials.

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In recent years, cases of inhumane treatment of foreigners, especially those from developing nations, at detention facilities have raised criticism from human rights groups.

The death last year of a Sri Lankan woman who was held in a detention center in Nagoya for overstaying her visa prompted her relatives to demand an investigation. Accounts by witnesses and information about her health condition suggested she was largely abandoned without adequate medical treatment.