Kofi Annan, the former UN secretary-general who won the Nobel Peace Prize for humanitarian work, has died aged 80, international diplomats say.
He "passed away peacefully on Saturday after a short illness", the foundation named after him said on Saturday.
Mr Annan was the first black African to take up the role of the world's top diplomat, serving from 1997 to 2006.
He later served as the UN special envoy for Syria, leading efforts to find a peaceful solution to the conflict.
In a statement announcing his death, the Kofi Annan Foundation described him as a "global statesman and deeply committed internationalist who fought throughout his life for a fairer and more peaceful world".
"Wherever there was suffering or need, he reached out and touched many people with his deep compassion and empathy. He selflessly placed others first, radiating genuine kindness, warmth and brilliance in all he did."
The diplomat, who was originally from Ghana, had been living in Geneva for several years before his death.
He was awarded the Nobel Peace Price in 2001 for helping to revitalise the international body, during a period that coincided with the Iraq War and the HIV/Aids pandemic.
Kofi Annan described his greatest achievement as the Millennium Development Goals which - for the first time - set global targets on issues such as poverty and child mortality.
However, Mr Annan was not immune to criticism. His critics blamed him for the UN's failure to halt the genocide in Rwanda in the 1990s when he was head of the organisation's peacekeeping operations.
Later, after the US-led invasion of Iraq, he and his son were accused of being involved in the "oil for food scandal" that led some to call for his resignation, though he was later exonerated.
In an interview with the BBC's HardTalk to mark his 80th birthday in April, Mr Annan acknowledged the UN's shortcomings, saying it "can be improved, it is not perfect but if it didn't exist you would have to create it".
"I am a stubborn optimist, I was born an optimist and will remain an optimist," he added.
Current UN Secretary-General António Guterres described Mr Annan as a "guiding force for good".
Kofi Annan was a guiding force for good. I join the world in mourning his loss. In these turbulent and trying times, his legacy as a global champion for peace will remain a true inspiration for us all. https://t.co/psJ9viPIeu pic.twitter.com/SKfBk5zaY2— António Guterres (@antonioguterres) August 18, 2018
UN high commissioner for human rights Zeid Raad Al Hussein said in a tweet he was grief-stricken over Mr Annan's death:
I am grief-stricken over the death of Kofi Annan. Kofi was the epitome of human decency and grace. In a world now filled with leaders who are anything but that, the world’s loss becomes even more painful. He was a friend to thousands and a leader of millions.— Zeid Raad Al Hussein (@raad_zeid) August 18, 2018
Tributes have been pouring in from world leaders and diplomats.
New Zealand Prime Minister Jacinda Ardern tweeted the world had lost a humanitarian leader.
The world has lost a great humanitarian, leader, and self proclaimed “stubborn optimist”. Rest in peace, Kofi Annan.— Jacinda Ardern (@jacindaardern) August 18, 2018
Former New Zealand Prime Minister and former director of the UN Development Program Helen Clark tweeted that she was deeply saddened by the news of Mr Annan's death saying that he was an "outstanding statesperson of our times". She offered condolences to his family and said "Kofi's life was one of service: he made a difference".
The co-chair of the European Council on Foreign Relations Carl Bildt described Mr Annan as a man of courage and wisdom.
Deeply saddened by the passing away of Kofi Annan. One of the true global leaders of our age. A man of courage, wisdom and friendship. I’m honored to have had the privilege of working for him. pic.twitter.com/a9pTv0cdF1— Carl Bildt (@carlbildt) August 18, 2018
Sad to hear of the death of Kofi Annan. A great leader and reformer of the UN, he made a huge contribution to making the world he has left a better place than the one he was born into. My thoughts and condolences are with his family. pic.twitter.com/P0SWagShJM— Theresa May (@theresa_may) August 18, 2018
Indian Prime Minister Narendra Modi said "the world has lost not only a great African diplomat and humanitarian but also a conscience keeper of international peace and security".
Mr Annan's diplomatic career continued after retirement from the UN, and in 2007 he set up his own foundation aimed at promoting global sustainable development, security and peace.
A year later, his reputation was boosted after he successfully helped negotiate a power-sharing deal to end post-election violence in Kenya.
In a Facebook tribute, Raila Odinga, the opposition leader who signed the agreement, called Annan "the man who stepped in and saved the country from collapse".
In 2012 he was made chair of The Elders, a peace and human rights advocacy group started by South Africa's Nelson Mandela.
The same year, he quit his post as UN envoy to Syria after only six months in the role, citing the failures of world powers to fulfil their commitments. He was later quoted as saying: "I lost my troops on the way to Damascus."
His most recent role was chairing an independent commission investigation Myanmar's Rohingya crisis.
Kofi Annan's wife, Nane, and three children were "by his side during his last days", the Kofi Annan Foundation said.
- BBC / RNZ