Flowers are placed under a portrait of the late former UN Secretary-General Kofi Annan at the UN headquarters in New York, on Aug. 18, 2018. Officials from across the UN system paid tribute to Kofi Annan, who passed away on Saturday at the age of 80 in a Swiss hospital. (Xinhua/Li Muzi)

BEIJING, Aug. 19 (Xinhua) -- World leaders have been mourning the death on Saturday of Kofi Annan, the Ghanian diplomat who was secretary-general of the United Nations (UN) from 1997 to 2006.

In a statement issued immediately after Annan's death, UN Secretary-General Antonio Guterres described him as "a guiding force for good", saying that "in many ways, Kofi Annan was the United Nations."

Michael Moller, director-general of the UN Office at Geneva, said: "Humanity has lost its strong moral voice, and I lost my mentor, my role model and great friend."

"He was an exceptional human being, with an amazing balance, infallible political instinct and ever present compassion, always caring for others, particularly the less fortunate of us," Moller said.

"Since the 1980s, when he worked in Geneva until his last years he remained an enduring voice on behalf of refugees and all who are oppressed," said UN High Commissioner for Refugees Filippo Grandi who hailed the charismatic international statesman as an "international leader, wise mentor, valuable adviser, good friend, role model."

Joining the UN in 1962, Annan spent much of his time engaging in humanitarianism-related work. He worked for the World Health Organization's Geneva office while in Switzerland, and went on to serve as UN under-secretary-general for peacekeeping operations between 1992 and 1996.

Zeid Ra'ad Al Hussein, UN high commissioner for human rights, said Annan was "irreplaceable" and "humanity's best example and the epitome of human decency and grace."

Audrey Azoulay, director-general of the United Nations Educational, Scientific and Cultural Organization (UNESCO), called Annan "a great defender of peace," saying that he was the very embodiment of peace and of a resolutely modern vision of the UN.

"His conviction that a culture of peace should be developed was fully in line with the mandate and daily commitment of UNESCO," she said.

"Mr. Annan spent his life advocating for peace and human dignity during his long career at the United Nations," U.S. Secretary of State Mike Pompeo said in a statement expressing his sadness over the death of Annan.

"Even after leaving his post as Secretary-General he embodied the mission of the United Nations, by sowing the seeds of peace as Chair of The Elders, an independent group of global leaders committed to advancing the cause of peace and promoting human rights around the world," he said.

In a statement carried by the Russian presidential press office, Russian President Vladimir Putin was quoted as saying: "I sincerely admired his wisdom and courage as well as his ability to make balanced decisions even under the most dire and critical circumstances."

"Russians will keep the memory of him forever," Putin added.

European Commission President Jean-Claude Juncker said that Annan, as an old friend and inspiration for him, had devoted his life to making the world a more peaceful and united place.

"His achievements as UN Secretary-General were rightly recognized with the Nobel Peace Prize and many other accolades throughout his life. But the greatest recognition we can give Kofi Annan is to keep his legacy and his spirit alive," he added.

German Chancellor Angela Merkel said in a statement that Annan shaped the UN "like hardly anyone before him." "He knew how to spark enthusiasm, particularly among young people."

Merkel added that "in our present time, in which the common search for solutions to global problems is more urgent than ever, we will greatly miss Kofi Annan's voice."

French President Emmanuel Macron said the world will never forget his calm and resolute approach to matters, nor the strength of his commitments. "France pays tribute to him," Macron tweeted.

Italian President Sergio Mattarella said in a statement that with Annan's passing, the international community lost a landmark figure, and the UN lost a former leader of peace, human rights and sustainable development.

British Prime Minister Theresa May wrote on Twitter: "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."

Describing Annan as a great peace builder, Finnish President Sauli Niinisto said the world has lost "a most staunch supporter of the rule-based international order."

"It was with great regret that I received information about the death of former UN Secretary General Kofi Annan; in our memory he will remain as a person who worked tirelessly to protect human rights and improve living conditions in Africa," Polish President Andrej Duda wrote in a condolence telegram.

Indian Prime Minister Narendra Modi also expressed his sorrow. "We express our profound sorrow at the passing away of Nobel laureate and former UNSG Kofi Annan. The world has lost not only a great African diplomat and humanitarian but also a conscience keeper of international peace and security," he tweeted.

"My thoughts are with his family and admirers in this hour of grief. May his soul rest in peace," he added.

In Annan's home country of Ghana, President Nana Akufo-Addo declared a week of national mourning starting on Monday, during which flags in the country and at its diplomatic missions would fly at half-staff.

"He brought considerable renown to our country by his position and through his conduct and comportment in the global arena ... He was an ardent believer in the capacity of the Ghanian to chart his or her own course onto the path of progress and prosperity," Akufo-Addo said in a statement.

"In this moment of sorrow, I condole with his family, relatives and friends. My thoughts and prayers go out to them as they come to terms with this sad news," Kenyan President Uhuru Kenyatta said in a statement.

Moussa Faki Mahamat, chairperson of the African Union Commission, said Annan was "a great man, a dear brother."

Annan was widely recognized for his role in advancing African agendas not only within the UN, but also after his retirement from the secretary-general's post.

For example, he has for many times called on the international community to step up efforts to combat HIV/AIDS, especially in sub-Saharan Africa where the epidemic was pandemic. He urged fresh funding to fight the disease at an African leaders summit in 2001.

Annan founded the Kofi Annan Foundation in 2007 to continue to promote better global governance and strengthen people's capacities to build a fairer and more peaceful world.

The Elders, an group gathering former leaders founded by late South African President Nelson Mandela, said they were "shocked and deeply saddened" by the death of Annan, who they said represented "a voice of great authority and wisdom in public and private."

"His quiet advice on how best to defuse impending crises was in constant demand from all corners of the globe, in particular from Africa," the group's deputy chair Gro Harlem Brundtland said.

An advocate for human dignity and international development, Annan, the first black African to head the UN, navigated the world body through one of its most testing times. He was awarded along with the UN the Nobel Peace Prize in 2001, shortly after the Sept. 11 terrorist attacks on the United States.

The Geneva-based Kofi Annan Foundation announced the former UN chief's death in Swiss capital Bern on Saturday. He was 80.