It was the 15th death since 2006 in a system widely criticized over medical standards, the monitoring of detainees and how guards respond to a medical emergency.

The man, in his 40s, died on Monday in the southern city of Nagasaki after he lost consciousness and was taken to hospital, said a detention center official who declined to be identified.

He did not give a cause of death.

RINK, a group supporting detainees at the center, told Reuters the Nigerian had been on hunger strike to protest his lengthy detention.

Another 27 foreigners are on hunger strike at a detention center in Ushiku, northeast of Tokyo, said a separate group supporting detainees at that facility.

Some of them have gone without food for 47 days, said Kimiko Tanaka, a spokeswoman for the group.

She said a 23-year-old Iranian man who sought asylum more than two years ago has lost weight and is using a wheelchair.

Two other men at Ushiku have been detained for five years, she said.

“The reality of a lengthy detention is nothing but a human rights violation,” Tanaka said.

An official at the national immigration agency confirmed there are hunger strikers at the Ushiku center, but he did not say how many. Authorities are providing medical care and trying to persuade them to eat, he added.

Immigration is a contentious issue in Japan, where ethnic and cultural homogeneity are deeply rooted.

Japan held about 1,500 detainees as of June 2018, according to the latest public data, nearly half of them for more than six months.

Some 604 were asylum seekers whose applications were rejected, while the rest were held for various immigration infractions such as overstaying visas.