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, without saying when he died.

During his tenure, 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's 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 health reasons.

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