Friday’s statement said Mandela, 94, who was discharged from hospital in April after receiving treatment for a lung infection, had suffered the same illness in the past few days.
It added that the Nobel laureate was receiving expert medical care and doctors were doing “everything possible to make him better and comfortable”.
President Jacob Zuma, on behalf of government and the nation, wished Madiba, Mandela’s clan name, a speedy recovery and requested the media and the public respect the privacy of the former president and his family.
Mandela, revered at home and abroad for leading the struggle against white minority rule, has been in and out of hospital for lung infection and other health problems.
Last year, he was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an acute respiratory infection.
In March and April, global figures such as US President Barack Obama sent him get-well messages and South Africans included Mandela in their Easter prayers.
Mandela, who became South Africa’s first black president in 1994 and served only one term in office, was jailed on Roben Island for 27 years for resisiting white minority rule.