South Africa's first black President Nelson Mandela

This is said to be the first time officials have revealed why Mandela, 94, was rushed to a military hospital in the capital, Pretoria, on Saturday.

News of the hospital stay has reportedly prompted much concern in South Africa.

Mandela is responding to his treatment, Mr Maharaj added.

Tests showed a "recurrence of a previous lung infection", presidential spokesman Mac Maharaj said.

The former president is revered at home and around the world for leading the struggle against white-minority rule before winning the country's first democratic elections in 1994.

In January 2011, he was treated for a serious chest infection.

He was last admitted to hospital in February when he was treated for abdominal pain.

He was awarded the Nobel Peace Prize in 1993.