UN nuclear watchdog chief Yukiya Amano has died, the International Atomic Energy Agency (IAEA) said on Monday, the day he had been expected to announce he would step down early because of an illness that visibly weakened him over the past year. 

"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. 

Amano had been preparing to leave his position in March, well before the end of his third four-year term, which ran until Nov. 30, 2021. Diplomats who follow the agency had said he planned to announce his decision on Monday.  

The IAEA announced last September that Amano had undergone an unspecified medical procedure. 

The 72-year-old Japanese diplomat 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 while seeking in vain to return to the Democratic People's Republic of Korea (DPRK). 

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 program in exchange for the lifting of sanctions.  

But tensions between Iran and the West have been rising since U.S. President Donald Trump unilaterally pulled out of the deal in May 2018. 

The IAEA said its flag over its headquarters in Vienna had been lowered to half-mast following Amano's death. 

Monday's statement did not lay out a time frame for naming a successor, though the 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. 

While each candidate will have their own management style, it is widely expected that there will be no major change in the agency's handling of its most high-profile issues, including Iran and a potential return to the DPRK, which expelled IAEA inspectors in 2009. 

(With input from Reuters, AFP) 

(Cover: International Atomic Energy Agency (IAEA) Director General Yukiya Amano leaves a news conference after a board of governors meeting at the IAEA headquarters in Vienna, Austria, June 8, 2015. /VCG Photo)