Hillary Clinton tweets 'love and prayers' to Nelson Mandela

Hillary Rodham Clinton has used her fifth tweet to send "love and prayers" to Nelson Mandela as he lies in critical condition in a Pretoria hospital.

The full Twitter post reads:

Madiba is Mandela's clan name.

President Barack Obama is due to visit South Africa with his family on Friday on a trip arranged before Mandela's health took a turn for the worse.

While SABC cited White House Press Secretary Jay Carney as saying their thoughts and prayers were with Mandela, USA Today reported that the president would not be visiting him in hospital.

The Mail & Guardian quoted South Africa International Relations Minister Maite Nkoana-Mashabane as telling reporters:

"President Obama would have loved to see Madiba. In my country, at my age and your age, when people are ill-disposed, we try to give them space to recover."

Meanwhile, Cape Town's Archbishop has led Mandela's closest family in a prayer asking for a "peaceful, perfect, end" for the anti-apartheid icon.

Thabo Makgoba consoled Mandela's wife Graca Machel and the family and in Pretoria Mediclinic Heart Hospital where Mandela has been for 18 days, slipping from "serious but stable" to "critical" condition.