Yukiya Amano in 2018. (Joe Klamar/AFP/Getty Images)

Yukiya Amano, the Japanese diplomat who led the International Atomic Energy Agency for a decade and was extensively involved in negotiations over Iran’s nuclear program and the cleanup of the 2011 Fukushima nuclear disaster, died July 18 at 72.

The Vienna-based agency announced the death but did not disclose where he died or provide a cause, adding that the news of his death was delayed at the request of his family. Mary Alice Hayward, the agency’s deputy director general and head of the department of management, is poised to lead the agency in the interim.

Mr. Amano, who had wide experience in disarmament, nonproliferation diplomacy and nuclear energy issues, had been chief of the key United Nations agency that regulates nuclear use worldwide since 2009.

The news of his death comes at a time of increasing concerns and escalating tensions over Iran’s nuclear program, after President Trump left a 2015 deal with world powers that restricted Iran’s nuclear program in return for sanctions relief. Mr. Amano was heavily involved in the years-long negotiations that led to the landmark Iran nuclear deal.

As head of the IAEA, Mr. Amano also dealt with the aftermath of Japan’s devastating Fukushima nuclear power plant accident in 2011, when three reactors went into meltdowns after a tsunami.

Mr. Amano’s death will be a blow for the nuclear agency, said Adnan Tabatabai, an expert with the Center for Applied Research in Partnership With the Orient in Bonn, Germany.

“While I am convinced that the IAEA as an institution will be able to continue its work dedicated to nuclear nonproliferation, the loss of a personality like Yukiya Amano, who had embodied this dedication, will add to an already highly delicate and complex situation with regards to the nuclear agreement with Iran,” he told the Associated Press.

Tabatabai suggested that opponents of the Iran nuclear agreement would “try to seize this opportunity to further weaken the position of the IAEA.”

The United States unilaterally withdrew from the accord a year ago, and Iran has recently inched past limits of its uranium enrichment set out in the accord. European leaders are scrambling to keep the accord intact.

Japan’s Foreign Ministry noted that after the Fukushima nuclear power plant meltdown, Mr. Amano led international efforts to improve nuclear safety based on the lessons learned from the accident while providing support for Japan.

Mr. Amano was Japan’s representative to the IAEA from 2005 until his election as director general in July 2009, including a stint as chair of its board of governors from 2005-2006.

Before he became IAEA chief, Mr. Amano contributed to the 1995, 2000 and 2005 Non-Proliferation Treaty Review conferences and chaired the 2007 preparatory committee for the 2010 Non-Proliferation Treaty Review Conference.

Mr. Amano was born in Yugawara, Japan, on May 9, 1947. A graduate of the Tokyo University Faculty of Law, he joined the Japanese Foreign Ministry in 1972 and was posted to jobs in Belgium, France, Laos, Switzerland and the United States. At the ministry, he was chief of the Disarmament, Non-Proliferation and Science Department from 2002 until 2005.

He also previously served as an expert on the U.N. panel on missiles and on the U.N. expert group on disarmament and nonproliferation education.

Survivors include his wife, Yukika.

— Associated Press