Retired Japanese diplomat Kenzo Oshima, who served as the undersecretary general of the United Nations and Japan's U.N. ambassador, died on May 29. He was 78.

Oshima fell ill on May 28 at his home in Tokyo and died of heart failure the following day, according to the Africa Society of Japan, where he served as the managing director.

Oshima was born in Hiroshima in 1943. After leaving the University of Tokyo in 1967, he joined the Foreign Ministry and assumed such posts as the head of the former Economic Cooperation Bureau and the U.N. undersecretary general for humanitarian affairs and emergency relief coordinator.

Oshima became Japan's ambassador to the United Nations in 2004. He pushed for Security Council reform to help Japan win a permanent seat on the council together with Germany and three other countries, but the plan was abandoned due to a failure to gain support from the United States.

When North Korea conducted a nuclear test in 2006, he urged members of the U.N. Security Council to adopt sanctions against the North.

Oshima was a survivor of the 1945 atomic bombing of Hiroshima, which occurred when he was 2 years old. He lost his mother in the attack.

He attended the Peace Memorial Ceremony in Hiroshima in 2001 as the U.N. undersecretary general. In his speech, he introduced himself as “one of the hibakusha” and read a message from U.N. Secretary-General Kofi Annan.

Oshima also engaged in efforts to support survivors of the Chernobyl nuclear accident at the United Nations.

After the triple meltdown at the Fukushima No. 1 nuclear power plant in 2011, he became a member of a Diet committee set up to investigate the nuclear accident and served as a commissioner of the Nuclear Regulation Authority, a government watchdog established after the Fukushima disaster.

Oshima was also a member of an independent commission investigating human rights abuses against Rohingya Muslims in Myanmar.

His funeral will be held with only his close relatives in attendance, according to the Africa Society of Japan.