Anti-apartheid icon Nelson Mandela started his third day in a Johannesburg hospital Friday, but the government said there was no need to panic over the health of the 92-year-old former South African president.

In a statement late Thursday, Deputy President Kgalema Motlanthe offered no specifics on why Mandela was taken to the hospital Wednesday, but said he was undergoing specialized tests. Mandela undergoes regular hospital checkups, but his latest visit has stretched into an unusually long stay.

Motlanthe referred to Mandela's history of respiratory problems. Mandela contracted tuberculosis during his 27 years in prison.

"There has been mounting concern about the health of the former President Nelson Mandela," Motlanthe said. "Medically there is no need to panic."

Motlanthe is scheduled to brief reporters on Mandela's health later Friday. He is acting president while President Jacob Zuma attended the World Economic Forum in Switzerland and then headed to a weekend African Union summit in Ethiopia.

His statement was the first substantive word on Mandela's condition since he entered Milpark Hospital Wednesday. Mandela's office released only a brief statement Wednesday, saying the visit was for routine tests and that Mandela was in "no danger and is in good spirits." The relative silence surrounding Mandela's hospitalization had led to speculation about his condition.

Journalists have been camped outside the hospital and outside his Johannesburg home. At the hospital, they have watched Mandela's relatives and friends enter for visits. Zuma was being updated while abroad by the defense minister, whose department is responsible for current and former presidents' health care, Zuma's office said.

The White House said President Barack Obama and first lady Michelle Obama's thoughts are with Mandela.

Mandela was jailed for 27 years for his fight against apartheid. He became South Africa's first black president in 1994 and stepped down after serving one term in 1999. He largely retired from public life in 2004.

The public has seen only glimpses of him recently, such as in November, when his office released photos of a private meeting between Mandela and members of the U.S. and South African soccer teams. The teams had just played a match in his honor.

Mandela also appeared at the closing ceremony of the World Cup in July, waving to the crowd as he was driven in a small golf cart alongside his wife, Graca Machel.