The United Nations nuclear watchdog on Saturday condemned the "disproportionate and unprecedented" move by Iran to withdraw accreditation from several of its most experienced inspectors.

Iran's foreign ministry said in response that the move was in retaliation for "political abuses" by the United States, France, Germany and Britain.

But IAEA director general Rafael Grossi said that this would seriously hamper the ability of the International Atomic Energy Agency to carry out its work.

"Today, the Islamic Republic of Iran informed me of its decision to withdraw the designation of several experienced Agency inspectors assigned to conduct verification activities in Iran" under an existing agreement, said Grossi.

"This follows a previous recent withdrawal of the designation of another experienced Agency inspector for Iran," his statement added.

"With today's decision, Iran has effectively removed about one third of the core group of the Agency's most experienced inspectors designated for Iran," said Grossi.

In 2015, major world powers reached a deal with Iran under which Tehran would curb its nuclear programme in exchange for relief from crippling economic sanctions.

But that started to unravel in 2018 when then US president Donald Trump unilaterally withdrew from the deal and reimposed sanctions.

Tehran in turn stepped up its nuclear programme -- while continuing to deny it harbours ambitions of developing a nuclear weapons capability.

Efforts to revive the deal have been fruitless so far.

- 'Arrogance' -

The United States and the so-called E3 group -- France, Germany and the United Kingdom -- are threatening to call for a new resolution against Tehran at an IAEA board meeting.

"Iran persists in its deliberate refusal to engage earnestly with the Agency," the US and E3 group said in a midweek joint statement.

If Iran did not fully abide by its obligations the Board would have to be prepared to take further action to support its Secretariat and hold Iran accountable, they added.

Iranian foreign ministry spokesman Nasser Kanani responded to the IAEA statement Saturday.

"Three European countries and the United States abused the space of the (IAEA's) Council of Governors for their own political purposes with arrogance and with the aim of destroying the atmosphere of cooperation.

"Of course, the Islamic Republic of Iran will continue its positive cooperation within the framework of the agreements made, emphasizing the necessity of the agency's neutrality," he added.

In his statement Saturday, Grossi said the experts affected had "unique knowledge in enrichment technology" and had previously conducted essential verification work at Iranian enrichment facilities under IAEA safeguards.

While the move was formally permitted under an existing agreement, Iran had done it "in a manner that affects in a direct and severe way the ability of the IAEA to conduct effectively its inspections in Iran," said Grossi.

"I strongly condemn this disproportionate and unprecedented unilateral measure which affects the normal planning and conduct of Agency verification activities in Iran and openly contradicts the cooperation that should exist between the Agency and Iran," he added

Without effective cooperation from Tehran, the agency would not be able to "provide credible assurances that nuclear material and activities in Iran are for peaceful purposes," Grossi stressed.