South Africa's former president Jacob Zuma, who was jailed for contempt of court last month, has been admitted to hospital, less than a week before he is due in court for a separate corruption trial.

Zuma, 79, is scheduled to attend the resumption of a long-running corruption trial on 10 August.

The hearing will include a plea to drop 16 charges of fraud, graft and racketeering against him related to the 1999 purchase of fighter jets, patrol boats and equipment from five European arms firms when he was deputy president.

He is accused of taking bribes from one of the firms, French defence giant Thales, which has been charged with corruption and money laundering.

Proceedings have been repeatedly postponed for more than a decade, sparking accusations of delaying tactics.

In a separate case, Zuma was handed a 15-month jail sentence in late June for snubbing a commission probing state corruption under his 2009-19 presidency.

He started serving the sentence on 8 July at Estcourt prison, in the east of the country.

Zuma's legal team had used a range of reasons to turn down invitations to testify, including alleged bias, preparations for the graft trial and medical concerns.

The former leader travelled to Cuba last year to receive treatment for an undisclosed illness.

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A routine observation at the prison prompted authorities to take him to an outside hospital for further examination, a Correctional Services statement said.

"Everyone who is detained, including every sentenced prisoner, has the right to conditions of detention that are consistent with human dignity, including ... medical treatment," the statement said.

It added that as a former president, Zuma's healthcare needs required the involvement of South African Military Health Services.

Zuma has already been allowed to leave prison for 24 hours to attend his brother's funeral last month.

Protests by his supporters broke out when Zuma handed himself in and escalated into riots involving looting and arson that President Cyril Ramaphosa described as an "insurrection".