* Foundation says admission routine check-up
* Zuma had argued jail would put him at great risk
* Zuma's graft trial set to resume on Tuesday (Adds more details, changes dateline to Johannesburg)
By Olivia Kumwenda-Mtambo
JOHANNESBURG (Reuters) - South Africa's jailed former president, Jacob Zuma, was taken to hospital for medical observation on Friday, prison authorities said, days before he was due to appear in court for a corruption trial.
Officials did not go into details on the condition of the 79-year-old, who is serving a 15-month sentence for contempt of court at Estcourt prison in KwaZulu-Natal province.
Zuma's foundation said he was in hospital for a routine annual check-up. "No need to be alarmed, ... yet," it said in a tweet on Friday morning.
Mzwanele Manyi, the spokesperson for the Jacob Zuma Foundation, told Reuters later in the afternoon the doctors were still seeing the former president and would advise him on whether he was fit to attend court on Tuesday.
The Department of Correctional Services said a routine observation at the prison had prompted authorities to take Zuma to an outside hospital for further examination. It did not name the hospital.
"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 department's statement said.
Zuma last month asked the Constitutional Court to annul his jail sentence, partly on the grounds that he was suffering from an unspecified medical condition.
He was jailed for defying a Constitutional Court order to give evidence at an inquiry investigating high-level corruption during his nine years in office until 2018.
When Zuma handed himself in on July 7, protests by his supporters escalated into riots involving looting and arson that President Cyril Ramaphosa described as an "insurrection."
Zuma, who was briefly permitted to leave jail on July 22 to attend the funeral of his younger brother, is due to appear in public again on Tuesday for an arms deal corruption trial.
In that case, he is accused of receiving kickbacks over a $2 billion arms deal from the 1990s. He pleaded not guilty in May to charges including corruption, fraud and money laundering.
Zuma has said he is the victim of a politically motivated witch-hunt. Efforts to prosecute him are seen as a test of South Africa's ability to hold powerful politicians to account. (Reporting by by Olivia Kumwenda-Mtambo; Writing by Toby Chopra; Editing by Jon Boyle, Giles Elgood and Andrew Heavens)