Kofi Annan, a former U.N. secretary general and Nobel Peace Prize winner for humanitarian work, died on Saturday at age 80 from an unspecified illness, the United Nations said.

Annan headed the United Nations for two terms from 1997 to 2006, a turbulent period due to the Sept. 11, 2001 terrorist attacks in the United States and the subsequent war in Iraq.

Annan, who died in Switzerland, was known for opposing the war in Iraq.

Annan and the world body were jointly awarded the Nobel Peace Prize in 2001 for their work contributing to a "more peaceful world," according to the Nobel Prize organization.

During his tenure as the secretary general, the advocate of human rights took a leading role in tackling the world's challenges including the battle against HIV/AIDS and global terrorism threat.

He also said the U.N. Security Council needed to be reformed to further reflect changes in the international community and supported the idea of Japan joining the permanent members of the Security Council.

The Ghanaian diplomat born on April 8, 1938, was also known for his deep commitment to the development of Africa, including his chairmanship of the Africa Progress Panel and early leadership of the Alliance for a Green Revolution in Africa, the Kofi Annan Foundation said.

He joined the U.N. system in 1962 and became the first U.N. chief to be elected from the ranks of its staff.

Before stepping down in December 2006, Annan was criticized for the responsibility he bore in overseeing the U.N. humanitarian aid program for Iraq when a "oil for food" corruption scandal emerged in 2004. Annan's son was implicated in the scandal.