At a press conference in Tokyo, Grossi had said Iran was "enriching (uranium) at 60 percent, so almost weapon level". He also described a 2015 nuclear deal with Iran as an "empty shell" that was "no longer fit for purpose".

Iran's Deputy Foreign Minister Kazem Gharibabadi lambasted Grossi's remarks as "unprofessional and totally politically charged", insisting his country's nuclear programme was solely for peaceful purposes.

Similarly, Iran's Atomic Energy Organisation said Grossi's remarks could be used as pretext by the United States and other Western countries "to exert unjust pressure on Iran".

In his remarks, Grossi said Iran had blown past the limits set by the 2015 deal, adding the goal currently was to prevent Iran from obtaining a nuclear weapon, or to help it "prove that they don't want to".

The deal -- known as the Joint Comprehensive Plan of Action -- collapsed following Washington's withdrawal from it in 2018 during the first term of US President Donald Trump, who returned to office in January.

Tehran continued to adhere to the deal for a year after Washington pulled out, but then began rolling back its commitments.

Iran has repeatedly expressed a willingness to revive the accord but efforts to that end have faltered.

The International Atomic Energy Agency (IAEA) nuclear watchdog, which Grossi leads, says Iran has increased its manufacturing of enriched uranium such that it is the only non-nuclear weapons state to possess uranium enriched to 60 percent.

That level is well on the way to the 90 percent required for an atomic bomb, according to the IAEA.

During his press conference, Grossi said that much of the 2015 deal was "no longer applicable", adding that the IAEA was ready to provide "technically sound alternatives to eliminate the possibility that Iran develops a nuclear weapon".

Tehran denies any intention of developing atomic weapons.

After his return to office, Trump reinstated his "maximum pressure" policy of sanctions against Iran, mirroring his approach during his first term.

He has also recently called for striking a new deal with Iran, but supreme leader Ayatollah Ali Khamenei has said "no problem will be solved by negotiating with America".