South African anti-apartheid icon Desmond Tutu, described as the country's moral compass, died on Sunday aged 90, President Cyril Ramaphosa said.

“The passing of Archbishop Emeritus Desmond Tutu is another chapter of bereavement in our nation's farewell to a generation of outstanding South Africans who have bequeathed us a liberated South Africa,” he said in a statement.

“Desmond Tutu was a patriot without equal; a leader of principle and pragmatism who gave meaning to the biblical insight that faith without works is dead,” he said.

“A man of extraordinary intellect, integrity and invincibility against the forces of apartheid, he was also tender and vulnerable in his compassion for those who had suffered oppression, injustice and violence under apartheid, and oppressed and downtrodden people around the world.”

A tireless activist, Tutu won the Nobel Peace Prize in 1984 for combatting white minority rule in his country.

Famously outspoken, even after the fall of the racist apartheid regime, Tutu never shied away from confronting South Africa's shortcomings or injustices.

It was Tutu who coined and popularised the term "Rainbow Nation" to describe South Africa when Nelson Mandela became the country's first black president.

However, in recent years, Tutu slammed even the ruling African National Congress (ANC) -- the vanguard of the fight against white-minority rule -- for cronyism and nepotism after apartheid ended in 1994.

In the past, he has confronted homophobia in the Anglican Church, challenged Mandela over generous salaries for cabinet ministers and stridently criticised the corruption that mushroomed under ex-president Jacob Zuma.

Ordained at the age of 30 and appointed archbishop in 1986, he used his position to advocate for international sanctions against apartheid, and later to lobby for rights globally.

Tutu was diagnosed with prostate cancer in 1997 and underwent repeated treatment.

He had retired a year earlier to lead a harrowing journey into South Africa's brutal past, as head of the Truth and Reconciliation Commission.

For 30 months, the commission lifted the lid on the horrors of apartheid.