JOHANNESBURG – Nelson Mandela’s health is improving, but the 94-year-old anti-apartheid hero remains in serious condition, South Africa’s president said Thursday.

Jacob Zuma paid a visit to Mandela in a Pretoria hospital, where the former president has been treated for six days, according to a statement released by the presidency.

“We continue to appeal to people to keep Madiba in their prayers and wish him a speedy recovery,” Zuma said, referring to Mandela by his clan name.

Mandela is being treated for a recurring lung infection, and it is his fourth hospitalization since December.