One of eight alleged South African mercenaries imprisoned in Equatorial Guinea is dying, his family said on Wednesday.

Bones Boonzaier, 48, was arrested with seven other South Africans in March. They are accused of being an advance party for a suspected coup in Equatorial Guinea. Boonzaier's wife, Georga, was in the country's capital, Malabo, after being granted a visa last week to visit her husband, who is being held in Black Beach Prison.

This was the first family visit granted since the men were arrested 18 weeks ago.

Boonzaier's son, Marius, said his father's condition had deteriorated to such an extent he only had about three weeks left to live.

"My father, who is chronically ill, has been physically abused and is in desperate need of medical attention," he said.

Boonzaier, speaking on behalf of his mother, said she had phoned him from Equatorial Guinea to give him the news.

"My mother told me my father is showing signs of terrible physical and mental abuse," he said.

Boonzaier said his father had claimed the prisoners were being abused by prison guards, who held AK 47 assault rifles to their heads.

"My father told my mother the guards torment and threaten to kill them," he said.

Bones Boonzaier also alleged that prison authorities injected all prisoners in need of medical attention with one intravenous needle.

Niek Du Toit, who allegedly confessed to the coup plot, was also reported to be showing signs of physical and mental abuse. According to Boonzaier, Du Toit was being held separately from the other prisoners.

Jody Kollapen of the South African Human Right's Commission called on the government to fulfil its obligations to its citizens imprisoned abroad.

"It is the government's responsibility to engage with the authorities in Equatorial Guinea with regard to the conditions of detention of the prisoners," he said.