The head of the global nuclear watchdog, Yukiya Amano, has died at the age of 72, the agency announced.
He has led the International Atomic Energy Agency (IAEA) since 2009, and was due to step down in March because of an unspecified illness.
During his tenure he had overseen a period of tense negotiations regarding Iran’s nuclear programme.
It is not yet clear who will succeed him, though discussions over his replacement began last week.
The Japanese national had taken over from Mohamed ElBaradei a decade ago and his third term was due to run until November 2021.
However, Amano appeared increasingly frail after undergoing an unspecified medical procedure in September.
“The Secretariat of the International Atomic Energy Agency regrets to inform with deepest sadness of the passing away of Director General Yukiya Amano,” the IAEA statement said.
EU foreign policy chief Federica Mogherini spoke of her sadness at Amano’s passing.
“A man of extraordinary dedication & professionalism, always at the service of the global community in the most impartial way. I’ll never forget the work done together. It has been for me a great pleasure & privilege working with him,” she tweeted.
The agency said the flag over its head office in Vienna had been lowered to half-mast.