Johannesburg, Dec 24 (IANS): South Africa's former president Nelson Mandela remained under medical treatment Sunday, the 15th day of his hospital stay, the presidency said.

This is Mandela's longest hospital stay since 2001, when he underwent seven weeks of radiotherapy after being diagnosed with prostate cancer, reported Xinhua.

In response to growing concerns about Mandela's health, presidential spokesman Mac Maharaj said there was "no new update" on Mandela's condition.

A large contingent of journalists have been camping outside the Mediclinic Heart Hospital in Pretoria, awaiting word on Mandela's condition although the government has never disclosed where Mandela was staying. Outside Mandela's residential house in Lower Houghton, Johannesburg, also a group of journalists have been camping a block away, expecting that Mandela would return home after being discharged from hospital.

On Saturday, President Jacob Zuma visited Mandela to assure him of the love and support of all South Africans, young and old, and the whole world. The presidency said after the visit that Mandela continued to respond to medical treatment.

In an earlier update on Mandela's condition, Zuma said Mandela's health condition was "serious" but he was "responding well to treatment" and had steadily improved in the last few days after suffering a lung infection and undergoing surgery for gallstones.

Mandela has been hospitalized in a Pretoria hospital since Dec 8 for a lung infection and gallstones. He underwent a successful surgery to have the gallstones removed after doctors attended to his lung infection.

Mandela, who was imprisoned for 27 years during apartheid, was elected the country's first black president in 1994 -- he remained president till 1999.