Yukiya Amano, an advocate for nuclear nonproliferation who played a key role in monitoring Iran’s compliance with the landmark 2015 nuclear deal as the head of the International Atomic Energy Agency, has died, the organization said in a statement on Monday. He was 72.
The Vienna-based organization provided few details but said the agency’s flags would be lowered to half-staff to honor Mr. Amano, who led the group for nearly a decade and was appointed in 2017 to a third four-year term.
Mr. Amano led the agency, a United Nations group commonly known by its initials, I.A.E.A., during a critical period in which it was responsible for overseeing Iran’s nuclear activities under the agreement reached in 2015. The United States abandoned the accord last year, and Iran has slowly been falling out of compliance with it in recent weeks.
The agency’s statement appeared to confirm reports from earlier this month that Mr. Amano would be stepping down from his position because of an unspecified illness. It included a message that Mr. Amano had intended to share in his letter to the organization’s governing board.
“During the past decade, the agency delivered concrete results to achieve the objective of ‘atoms for peace and development,’ thanks to the support of member states and the dedication of agency staff,” he wrote. “I am very proud of our achievements, and grateful to member states and agency staff.”
Seyed Abbas Araghchi, Iran’s deputy foreign minister, said on Twitter on Monday that he had worked closely with Mr. Amano, who oversaw the monitoring of Iranian nuclear energy development after the 2015 nuclear deal was signed.
“I commend his skillful & professional performance,” as the head of the agency, Mr. Aragchi wrote on Twitter.
John Bolton, President Trump’s national security adviser, said in a statement that Mr. Amano’s death was a “great loss for Japan, the United States, and to many, many people from around the world.”
Mr. Bolton made his remarks during a visit to Japan, where he was expected to meet with officials and discuss the United States’ tensions with Iran.
“His commitment to nuclear nonproliferation and his championing of peaceful nuclear energy have been unparalleled,” Mr. Bolton said of Mr. Amano. “I was proud to call Yukiya a friend and colleague. He will be sorely missed.”
Karin Kneissl, a former foreign minister of Austria, said that while many people within the I.A.E.A. had made it possible for the agency to play a role in monitoring Iran’s compliance to the 2015 nuclear deal, Mr. Amano’s discreet and diligent approach had been crucial in the current climate of animosity between Washington and Tehran.
“Mr. Amano’s role — I would say particularly ever since the United States withdrew in 2018 — was his sober and professional handling of the whole issue,” she said.
With the agency acting as “the guardian of this agreement,” as she put it, maintaining a technical approach rather than getting involved in the politics of the matter was key.
“He was not just a man of the office, he always went out and in that way he was different from his predecessors,” she noted. “As the director general, you carry the final responsibility, and it’s not an easy task because you are under scrutiny.”
Mr. Amano graduated from the Tokyo University Faculty of Law in 1972. He was the first Asian person to lead the I.A.E.A., ascending to that position after serving in the Japanese Foreign Ministry for more than three decades in a variety of roles.
Mr. Amano was a strong advocate against nuclear proliferation, and his views were shaped by the fact that his country is the only one to have experienced the devastation of having such weapons used on its soil.
“He lent an ear to survivors of atomic bombs in Hiroshima and Nagasaki, and urged that ‘nuclear weapons should be abolished,’ ” the Japanese daily Sankei Shimbun reported in a 2009 article.
While serving as the head of the nuclear agency, Mr. Amano also oversaw safety efforts after the disaster at Fukushima Daiichi nuclear power plant in 2011 in his home country after an earthquake and a tsunami battered the facility causing a meltdown.
Four years after the episode, Mr. Amano delivered a statement to the agency’s governing board and said he believed the Fukushima accident had brought nuclear safety into sharper focus.
“There can be no grounds for complacency about nuclear safety in any country,” he told the board at the time. “Continuous questioning and openness to learning from experience are key to a safety culture and are essential for everyone involved in nuclear power. Safety must always come first.”
Taro Kono, Japan’s foreign minister, said in a statement on Monday that Mr. Amano had not only “vigorously addressed international nuclear nonproliferation issues” but also had helped develop the peaceful use of nuclear power.
During his tenure, Mr. Amano also established a team to prepare for the resumption of I.A.E.A. inspections in North Korea if an agreement were reached between Washington and Pyongyang, the state news outlet NHK reported.
Federica Mogherini, the European Union’s top foreign policy official, wrote on Twitter that she was “saddened by the loss” of Mr. Amano.
“A man of extraordinary dedication & professionalism, always at the service of the global community in the most impartial way,” she wrote. “I’ll never forget the work done together. It has been for me a great pleasure & privilege working with him.”