WASHINGTON - US President Barack Obama on Tuesday spoke highly of Nelson Mandela, a day before the former South African president turns 94.

In a statement, Obama and his wife Michelle extended their "heartfelt congratulations" to Mandela on the occasion of his 94th birthday and the fourth annual Nelson Mandela International Day.

"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," Obama said.

"Nelson Mandela's personal story is one of unbreakable will, unwavering integrity, and abiding humility," he said, adding "On a personal note, our 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."

Madiba is Mandela's Xhosa clan name.

In her visit to South Africa on June 21, 2011, Michelle, along with her mother, two daughters, niece and nephew, spent some 20 minutes with Mandela at his home in Houghton in northern Johannesburg.

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

In 2009, the United States joined 192 United Nations member states in the creation of Nelson Mandela International Day.

"There is no more fitting tribute to a man who has demonstrated to the world the extraordinary power of non-violence, of tolerance, and of unwavering service to our fellow men and women," Obama said.

Mandela was imprisoned for 27 years during apartheid, and was elected South Africa's first black president in 1994. He left office after serving his five-year term.