Grossi also said the situation at the Ukrainian nuclear power plant of Zaporizhzhia was "extremely worrying", even if there are no signs the plant has become a military installation.

The UN agency, based in Vienna, has been struggling since 2021 to carry out controls on Iran's nuclear programme, which continues to expand even as Tehran denies it wants to make nuclear weapons.

"It's a very frustrating situation. We continue our activities there, but at a minimum," Grossi said in an interview at the World Economic Forum in Davos. "They are restricting cooperation in a very unprecedented way."

He cited as an example Iran's rejection of inspectors because of their nationalities.

"It's a way to punish us because of external things," he said. "When there's something that France, the UK or the United States says that they don't like, it is as if they were taking the IAEA hostage to their political disputes with others. This is unacceptable for us."

- 'Diplomacy, diplomacy, diplomacy' -

Iran last year slowed down its pace of uranium enrichment, which was seen as a goodwill gesture while informal talks began with the United States. But it accelerated enrichment once again in late 2023.

"There is a plateau at the moment, but it could change in the next few days," Grossi said. "We never know."

The already poor relations between Washington and Teheran have worsened with the conflict between Israel and Hamas, with each nation accusing the other of inflaming the situation, further complicating efforts to reign in Iran's nuclear programme.

"Diplomacy, diplomacy, diplomacy, this is what we need. We need to continue talking, we need to prevent the situation deteriorating to a degree where it would be impossible to retrieve it."

"I would not exclude returning to Iran," Grossi added.

- 'No militarisation' of Zaporizhzhia -

IAEA inspectors have also been blocked by Russia from visiting the Zaporizhzhia nuclear plant, which has been occupied by Russian forces since March 2022, soon after the start of Russia's invasion of Ukraine.

The plant is within a combat zone, which makes it structurally unstable, and Grossi said his inspectors needed to be granted access.

"We have been able to confirm that there is no militarisation of the plant, in the sense of having heavy military equipment or artillery equipment there," he said. "And in the past few months, there haven't been any direct attacks on the plant."

On the other hand, "we've had problems in terms of blackouts, and interruption of external power supply, which are equally dangerous because if we lose power, we lose the capacity to cool the reactors and of course, there could be an accident."