A Vietnamese man who died in solitary confinement last month at the East Japan Immigration Centre in Ibaraki Prefecture succumbed to a stroke, a Justice Ministry official said Monday.

The death of Nguyen The Hung raised the death toll in Japan's detention system to 13 since 2006.

The deaths have provoked criticism from activists and a watchdog overseeing the centers about the quality of medical care provided to detainees and how they are monitored.

An autopsy found that Nguyen died of a subarachnoid hemorrhage, the official said, requesting anonymity. The autopsy was carried out on Wednesday, a separate person with direct knowledge of the matter said.

While the full details of Nguyen's medical condition and treatment at the center remain unclear, fellow detainees said he had repeatedly told guards he was suffering from pain in his head and neck since his arrival.

Nguyen was prescribed painkillers by a doctor at the center, the detainees said in a letter, only for guards to ignore his later complaints of pain because he was held in solitary and told him to be quiet.

Toru Tsunoda, a doctor and vice chairman of a watchdog overseeing conditions at the center, said that although strokes are hard to detect, guards may have missed symptoms of Nguyen's illness.

"They may have been unable to detect the stroke by only monitoring Nguyen in the solitary cell," Tsunoda said.

Nguyen, born in 1969, arrived in Japan in 1998 to seek asylum, a Vietnamese nun said. He was one of more than 11,000 refugees Japan took in during the aftermath of the Vietnam War.

A Reuters investigation into the death of a Sri Lankan held in a solitary cell at a Tokyo detention center revealed serious gaps in medical care and checks on ill detainees.