JOHANNESBURG (AFP) - Former South African president Nelson Mandela, 94, was readmitted to hospital on Saturday with a renewed lung infection and is in "serious but stable" condition, the presidency said.

"This morning at about 1:30 am (7:30 am in Singapore) his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition," President Jacob Zuma's office said in a statement.

It marks the second hospitalisation for the ailing anti-apartheid hero in two months.

On April 6 he was released from hospital after being treated for pneumonia during a 10-day stay.