Iran's nuclear activities are reaching "new heights" as the country continues to enrich uranium with "no credible peaceful purpose", western diplomats said.
A slight reduction in Iran's stockpile of 60 per cent enriched uranium should not give "false hope" because it could soon be replenished, the International Atomic Energy Agency was told.
A statement by the UK, France and Germany to the watchdog's board said Iran had installed more advanced centrifuges and "laid ground for a further expansion" of its ability to enrich uranium.
Its overall stockpile has grown by 30 per cent, the statement said, although Iran appeared to have downblended some of its stock of 60 per cent uranium to 20 per cent. The reason for this is not clear.
Uranium of 60 per cent purity is considered a short step from the 90 per cent level regarded as weapons-grade material. A level of 3 per cent to 5 per cent is normal for civilian nuclear power stations.
Iran denies it is trying to build a nuclear weapon, but has raised alarms at the IAEA by obstructing inspections and failing to account for uranium traces at undeclared sites.
Tehran no longer abides by limits on uranium enrichment it agreed to under a 2015 deal with world powers, which the US abandoned in 2018.
"Iran has pushed its nuclear activities to new heights that are unprecedented for a state without a nuclear weapons programme," the statement delivered by Germany on behalf of the three European powers said.
"The recent slight reduction of the stockpile of uranium enriched up to 60 per cent should not lead us to false hope and wrong conclusions.
"Enrichment up to 60 per cent has continued unabated, increasing rates of production will soon see the stockpile increase again, and downblending has amplified the sharp increase of the stockpile of uranium enriched up to 20 per cent."
A separate statement by Laura Holgate, US ambassador to the IAEA, said Iran's nuclear activities "remain deeply troubling" and that its enrichment has "no credible civilian purpose".
"We continue to have serious concerns related to the stockpile of highly enriched uranium that Iran continues to maintain," she said.
"No other country today is producing uranium enriched to 60 per cent for the purpose Iran claims and Iran’s actions are counter to the behaviour of all other non-nuclear weapons states party to the NPT [non-proliferation treaty].
"Iran should downblend all, not just some, of its 60 per cent stockpile, and stop all production of uranium enriched to 60 per cent entirely."
