"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 the NPT Safeguards Agreement," IAEA director general Rafael Grossi said in a statement.

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

"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," added Grossi, saying his affected colleagues had "unique knowledge in enrichment technology" who had previously conducted essential verification work at Iranian enrichment facilities under IAEA safeguards.

"This measure, while formally permitted by the NPT Safeguards Agreement, has been exercised by Iran 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.

"Without effective cooperation, confidence and trust will continue to be elusive and the Agency will not be in a position to discharge effectively its verification mandate in Iran and provide credible assurances that nuclear material and activities in Iran are for peaceful purposes."

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.

The United States and the so-called E3 group -- France, Germany and the United Kingdom -- are now 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, warning that if Iran does not fully abide by its obligations the Board will have to be prepared to take further action to support its Secretariat and hold Iran accountable.