The Director-General of the International Atomic Energy Agency (IAEA), Yukiya Amano, died at the age of 72, the agency announced Monday.

Having led the agency since 2009, the former Japanese diplomat oversaw various major events including cleanup of the 2011 Fukushima nuclear disaster and negotiations over Iran’s nuclear weapons program.

“The Secretariat of the International Atomic Energy Agency regrets to inform with the deepest sadness of the passing away of Director General Yukiya Amano,” the IAEA said in a statement Monday.

Amano’s death happened well before the end of his term, due to run until November 2021. He was planning to step down after becoming increasingly frail following an unspecified medical procedure last September.

The agency revealed that he was preparing to send a letter to the board of governors explaining his decision retire, part of which was released to the public.

“During the past decade, the Agency delivered concrete results to achieve the objective of ‘Atoms for Peace and Development’ thanks to the support of the Member States and the dedication of Agency staff,” Amano wrote. “I am very proud of our achievements and grateful to Member States and Agency staff.”

Amano reportedly passed away on Thursday, although his family informed the IAEA late on Sunday “with the specific request not to disclose it until the family funeral had taken place on 22 July in a quiet atmosphere.”

White House National Security Advisor John Bolton was one of many world figures to pay tribute to Amano, describing his passing as a “great loss for Japan, the United States, and to many, many people from around the world.”

“His commitment to nuclear nonproliferation and his championing of peaceful nuclear energy have been unparalleled in leading the International Atomic Energy Agency for almost a decade,” Bolton said in a statement. “I was proud to call Yukiya a friend and colleague. He will be sorely missed. On behalf of President Trump and the United States, my deepest condolences extend to the family and friends of Yukiya at this difficult time.”

Tributes also came in from the European Union, with its foreign policy chief Federica Mogherini describing him as “a man of extraordinary dedication & professionalism, always at the service of the global community in the most impartial way.”

So saddened by the loss of @iaeaorg DG #Amano 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 — Federica Mogherini (@FedericaMog) July 22, 2019

Iran’s Deputy Foreign Minister Seyed Abbas Araghchi praised his “skillful & professional performance” in facilitating negotiations for former American President Barack Obama’s 2015 nuclear deal with Iran, known officially as the “Joint Comprehensive Plan of Action.” Araghchi took the opportunity to note that, under Amano, the IAEA issued “15 consecutive reports confirming Iran’s full compliance to the [The Joint Comprehensive Plan of Action].”

My deep condolences for #Amano demise. We worked very closely. I commend his skillful & professional performance as DG of IAEA resulted in complete closure of so-called #PMD & 15 consecutive reports confirming Iran's full compliance to the #JCPOA. May the Almighty bless his soul. pic.twitter.com/tyTDQ9AiTY — Seyed Abbas Araghchi (@araghchi) July 22, 2019

It is currently unclear who will replace Amano as head of the agency. Some of the possible contenders include Argentina’s ambassador to the IAEA, Rafael Grossi, and the agency’s chief coordinator, Romanian Cornel Feruta.

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