The head of the World Health Organization (WHO) has died, two days after emergency surgery for a blood clot on his brain.
Lee, a TB expert, spearheaded the fight against bird flu
Lee Jong-wook, 61, was rushed to hospital in Geneva on Saturday. He had been described as "recovering" but he died early on Monday.
Dr Lee, from South Korea, had worked at the WHO for 23 years and became its head in 2003.
The news came as the 192-nation body convened its annual assembly.
"I am sorry to tell you that Dr Lee Jong-wook, director-general of the WHO, died this morning," Spain's Health Minister Elena Salgado told the opening session of the assembly.
It is a "very sad bit of news", said Ms Salgado, who is chairing the assembly. Dr Lee "was an exceptional person and an exceptional director-general", Ms Salgado said.
The assembly observed a two-minute silence and was suspended for 30 minutes following the announcement.
Dr Lee was taken ill on Saturday afternoon while attending an official function. He was taken to Geneva's Cantonal Hospital and underwent immediate emergency surgery for a blood clot in his brain.
Dr Lee was thought to be recovering from the operation, but he died at 0743 (0543 GMT) on Monday morning, said a statement on the WHO website.
"All of the staff of the World Health Organization extend their most sincere condolences to Dr Lee's family. The sudden loss of our leader, colleague and friend, is devastating," said the statement.
"Dr Lee led WHO to continue its mission to help people attain the highest possible level of health."
UN Secretary General Kofi Annan said in a statement: "I am shocked and deeply saddened to learn of the sudden passing of Dr Lee.
"This sudden loss of a leader, colleague and friend is truly devastating."
Dr Lee was the first South Korean to head a UN agency. As head of the agency's tuberculosis programme, he was admired for his understated but efficient management style.
He had said his tenure as director-general would be defined by the fight against Aids. In fact his time in office was dominated by bird flu and the fear it could mutate into a strain spread from person to person.
Dr Lee is survived by his wife and son.