The longtime Japanese diplomat, who was 72, held the IAEA's top job since December 2009.

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

It said that Amano died July 18. But his family had only informed the IAEA late Sunday, "with the specific request not to disclose it until the family funeral had taken place on 22 July in a quiet atmosphere."

No details were given about the cause of death.

During his 10 years at the helm, Amano oversaw the signing of a landmark deal in 2015 between Iran and six major powers -- Britain, China, France, Germany, Russia and the United States -- under which the Islamic republic agreed to curb its nuclear programme in exchange for the lifting of sanctions.

But international tensions with Iran have been rising since US President Donald Trump unilaterally pulled out of the deal in May 2018.

Amano took over from Egypt's Mohamed ElBaradei in 2009. And his third term as IAEA chief had originally been due to expire in November 2021, but he had been expected to announce his intention to step down early because of undisclosed health reasons.

The IAEA said its flag over its headquarters in Vienna had been lowered to half-mast.

Iran and the US quickly paid tribute to the Japanese diplomat.

Iranian Deputy Foreign Minister Abbas Araghchi expressed his "deep condolences" in a tweet, saying: "I commend his skillful & professional performance as DG of IAEA... May the Almighty bless his soul."

Washington's ambassador to the international organisations in Vienna, Jackie Wolcott, said the United States "and all nuclear non-proliferation advocates have lost a great friend, and the United Nations family has lost an exceptional public servant."

In Moscow, Russian President Vladimir Putin said he "always admired his wisdom and foresight -- his ability to make informed decisions in the most difficult circumstances."

- Escalating tensions -

The IAEA, which is tasked with monitoring Iran's nuclear activities to ensure they abide by the terms of the 2015 deal known as the Joint Comprehensive Plan of Action, will now have to find Amano's successor as tensions between Washington and Tehran escalate ever higher.

In retaliation to Trump's withdrawal from the JCPOA, Iran announced earlier this month that it no longer felt bound by some of the terms of the pact.

It said it would breach one of the limits on stockpiles of nuclear material and also start to enrich uranium at a higher level than agreed -- both of which has since been confirmed by the IAEA.

Against this backdrop of brinkmanship, Iran's Islamic Revolutionary Guard Corps confiscated a UK-flagged tanker in the Strait of Hormuz on Friday, two weeks after British authorities seized an Iranian tanker off Gibraltar.

The IAEA's director general is chosen by the 35 member states that sit on its board of governors.

The agency has 171 member states in all and plays a key role in the fight against nuclear proliferation, by verifying that signatories to the Treaty on the Non-Proliferation of Nuclear Weapons abide by their commitments.

Among those mooted as his possible successor are the IAEA's chief coordinator, Cornel Feruta, a Romanian, as well as Argentina's envoy to the agency, Rafael Grossi.

Experts suggest that the appointment process could become deeply politicised given the current international climate.

Mark Hibbs, a research fellow at Carnegie Endowment for International Peace, said the IAEA would "want to limit the extent to which the election ventures into political areas that are remote from the business of the agency".