UN nuclear watchdog chief ­Yukiya Amano has died, the International Atomic Energy Agency (IAEA) said Monday, just as he was preparing to step down because of an unspecified illness.

The 72-year-old Japanese had held the position of IAEA director-general since 2009, taking over from Mohamed ElBaradei and steering the UN agency through a period of intense diplomacy over Iran's nuclear program. He had been preparing to leave his position in March, well before the end of his third four-year term, which ran until November 30, 2021.

The IAEA announced in September that Amano had undergone an unspecified medical procedure. The specific nature of his illness has remained a taboo subject within the agency, diplomats say, but with each public appearance he had appeared increasingly frail.

"The Secretariat of the International Atomic Energy Agency regrets to inform with deepest sadness of the passing away of director-general Yukiya Amano," the secretariat's note read.

The note did not lay out a timeframe for naming his successor, though a race to succeed him had been taking shape since last week, when it became clear he would step down early.

Argentina's Ambassador to the IAEA, Rafael Grossi, is running to succeed Amano, and diplomats say the agency's chief coordinator Cornel Feruta of Romania, effectively Amano's chief of staff, is likely to run. Others could also enter the fray.