US President Barack Obama and his wife, Michelle Obama, paid tribute to South African ex-president Nelson
Mandela ahead of his 94th birthday, as having "abiding humility" and "unbreakable will".

"On behalf of the people of the United States, we would like to extend our heartfelt congratulations to Nelson Mandela on the occasion of his 94th birthday and the fourth annual Nelson Mandela International Day," the US first couple said in a statement honouring the anti-apartheid icon on Tuesday.

Mandela's "extraordinary life and steadfast commitment to the principles of democracy and reconciliation continues to be a beacon for people of all backgrounds who strive for dignity, justice, and freedom," they said, describing his personal story as "one of unbreakable will, unwavering integrity, and abiding humility".

The Obamas said their family "has been inspired by Madiba's example, and has deeply appreciated the time we have spent with him, and his wisdom, grace and generosity of spirit".

"By any measure, Nelson Mandela has changed the arc of history, transforming his country, continent, and the world," the statement read.

Clinton visits

Obama's Democratic predecessor Bill Clinton, meanwhile, hailed his close ties with Mandela before visiting the South African icon's home in his childhood village.

Clinton spent two hours inside Mandela's family compound in Qunu, the picturesque south-eastern village where the South African leader grew up.

Media were barred from entering the area and Clinton left without speaking.

Before the visit Clinton opened a library at a primary school together with Mandela's wife, Graca Machel, and daughter Zindzi ahead of the statesman's birthday on Wednesday.

"We worked together as presidents and even after we left office we continued working together to improve education of the children worldwide in order for them to share the future," Clinton said.

Mandela, who was president from 1994-1999, spent 27 years in the apartheid regime's jails. He retired from political life in 2004.