Countries on opposite sides of current global tensions have been paying tribute to the International Atomic Energy Agency's director general, Yukiya Amano who's died at 72.
Amano, a former Japanese diplomat, had extensive experience in disarmament and non-proliferation diplomacy, as well as nuclear energy issues, and had been chief of the key agency that regulates nuclear issues worldwide since 2009.
The news of his death comes at a time of escalating concern about Iran's nuclear program after the United States left a 2015 deal with world powers that restricted the country's nuclear uranium enrichment.
Amano was heavily involved in the long negotiations that led to the Iran nuclear treaty.
The IAEA said its flag will be lowered to half-mast.
The agency said he'd been planning to write soon to the agency's board of governors announcing his decision to step down. It released part of a letter, in which he praised the agency for delivering "concrete results to achieve the objective of 'Atoms for Peace and Development' plan."
Iran has expressed its "deep condolences" over the death of the nuclear expert diplomat.
Deputy Foreign Minister Abbas Araghchi commended Amano's skillful and professional performance in his role.
"May the Almighty bless his soul", Araghchi writes.
John Bolton, President Donald Trump's national security adviser, said in a statement released by the US embassy in Tokyo, that Mr Amano's "commitment to nuclear non-proliferation and his championing of peaceful nuclear energy have been unparalleled in leading the International Atomic Energy Agency for almost a decade. He will be sorely missed".
The Tokyo Electric Power Co, which ran the nuclear plant in Fukushima, northeastern Japan, also praised him, saying it "received so much support and guidance on the decommissioning efforts" at the power plant from him.
German Foreign Minister Heiko Maas says Amano made the IAEA stronger in difficult times.
"General Director Amano's tireless dedication to strengthening the nuclear Non-Proliferation Treaty in the United Nations Security Council will always stay with me personally," Maas said.
Australian Associated Press