The International Atomic Energy Agency has announced the death of its director general Yukiya Amano at the age of 72.
Officials did not say how Mr Amano died.
The IAEA’s Secretariat said he was planning to write soon to the board of governors announcing his decision to step down.
In that letter, he praised the agency for delivering “concrete results to achieve the objective of the ‘Atoms for Peace and Development’ plan”.
He added that he was “very proud of our achievements and grateful” to member states and agency staff.
The EU’s foreign policy chief Federica Mogherini said she was saddened by Mr Amano’s death and called him “a man of extraordinary dedication and professionalism, always at the service of the global community in the most impartial way”.
According to the agency’s biography, Mr Amano was Japan’s representative to the agency from 2005 until his election as director general in July 2009, including a stint as chair of its board of governors from 2005-2006.
He had extensive experience in disarmament and non-proliferation diplomacy, as well as nuclear energy issues, and assumed his duties as IAEA director general on December 1 2009.
A graduate of the Tokyo University Faculty of Law, Mr Amano joined the Japanese foreign ministry in 1972 and was posted to jobs in Belgium, France, Laos, Switzerland, and the United States.
At the foreign ministry, Mr Amano was chief of the Disarmament, Non-Proliferation and Science Department from 2002 until 2005.
He previously served as a governmental expert on the UN Panel on Missiles and on the UN Expert Group on Disarmament and Non-Proliferation Education.
Mr Amano contributed to the 1995, 2000 and 2005 Nuclear Non-proliferation Treaty Review Conferences, and he chaired the 2007 Preparatory Committee for the 2010 Nuclear Non-proliferation Treaty Review Conference.