FILE - In this Nov. 22, 2018, file photo, International Atomic Energy Agency (IAEA) Director General Yukiya Amano of Japan addresses the media during a news conference after a meeting of the IAEA board of governors at the International Center in Vienna, Austria. The IAEA said Monday, July 22, 2019, it is announcing with regret the death of Amano. The Secretariat did not say how Amano, who was 72, died. (AP Photo/Ronald Zak)

VIENNA (AP) — 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 power plant disaster, has died at 72, the agency announced today.

Amano, who had wide experience in disarmament, non-proliferation diplomacy and nuclear energy issues, had been chief of the key U.N. 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 U.S. President Donald Trump left a 2015 deal with world powers that restricted Iran’s nuclear program in return for sanctions relief. Amano was heavily involved in the yearslong negotiations that led to the landmark Iran nuclear deal.

As head of the IAEA, Amano also dealt with the aftermath of the devastating 2011 Fukushima nuclear accident in Japan, where three reactors sank into meltdowns after a tsunami.

The IAEA Secretariat did not give a cause of death for Amano, or say where or when he died.

The IAEA said Mary Alice Hayward, the agency’s deputy director general and head of the department of management, would lead the agency as acting director general. The IAEA flag will be lowered to half-mast in tribute to its leader.

The agency said Amano was planning to write soon to its board of governors announcing his decision to step down. It released part of that letter, in which Amano praised the agency for delivering “concrete results to achieve the objective of ‘Atoms for Peace and Development.'” Amano added that he was “very proud of our achievements and grateful” to IAEA member states and agency staff.

Amano’s death will be a strong 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 non-proliferation, 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 nuclear agreement would “try to seize this opportunity to further weaken the position of the IAEA.”

“It is therefore of upmost importance that the agency immediately gets its act together and presents Amano’s successor, who will have to come out in strong support” of the Iran nuclear deal, he said.

Tributes to Amano poured in Monday from around the world, including Japan, Iran, the European Union and the United States.

Iran’s Deputy Foreign Minister Seyed Abbas Araghchi expressed his condolences, writing on Twitter that they had worked closely together and commending his professionalism. He said they had ensured Iran’s compliance with the JCPOA, the nuclear accord that Iran struck with world powers in 2015 that is now unraveling under pressure from the Trump administration.

The U.S. 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.

The European Union’s foreign policy chief, Federica Mogherini, called Amano in a tweet “a man of extraordinary dedication & professionalism, always at the service of the global community in the most impartial way.”

Germany’s Foreign Minister Heiko Maas said Amano “made the IAEA stronger.”

Maas praised Amano, saying the agency’s inspection of the Iran nuclear deal was an example of Amano’s “biggest-possible dedication, professionalism and independence.”

Germany is one of the nations that signed the 2015 Iran nuclear deal and is now scrambling to salvage it.

Jackie Wolcott, the U.S. ambassador to international organizations in Vienna, said Amano “was greatly respected as an effective leader, diplomat, and true gentleman by the entire staff of the U.S. Mission and other U.S. diplomats.”