"My deep condolences... We worked very closely," tweeted Araghchi, who took part in the negotiations for a hard-won nuclear deal agreed between Iran and world powers in 2015.

"I commend his skillful and professional performance... (that) resulted in complete closure" of the file on the "possible military dimension" of Iran's nuclear programme.

"May the Almighty bless his soul," he wrote in English.

Amano died after suffering poor health for some time, the atomic agency said.

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

During his tenure, Amano oversaw the signing of the landmark deal between Iran and six major powers -- Britain, China, France, Germany, Russia and the United States.

Under the deal, the Islamic republic agreed to curb its nuclear programme in exchange for the lifting of sanctions.

In December 2015, the IAEA said it "has no credible indications of activities in Iran relevant to the development of a nuclear explosive device after 2009".

In his tweet, Araghchi noted that under Amano the IAEA had issued "15 consecutive reports confirming Iran's full compliance" with the nuclear accord.

However, since the beginning of July, the IAEA has noted that Iran failed in two of its commitments by no longer respecting limit imposed by the agreement.

Iran announced this month that its enriched uranium stockpile had exceeded the 300 kilogrammes imposed by the accord.

Days later it said it would begin enriching uranium to above the 3.67 percent level set by the deal, announcing the following day it had reached 4.5 percent.