The 94-year-old anti-apartheid hero and the country's first black president has been in hospital three times since December and has been battling this bout of the infection for several days, it said.
"This morning at about 1:30 a.m. his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition," the government said.
Presidential spokesman Mac Maharaj told local television Mandela's condition was "serious this time".
"The situation is serious this time but doctors have assured us he is comfortable," he told television station eNCA. DM
Photo by Greg Marinovich.