Advanced centrifuges at Iran's Natanz facility "were accumulating, or had been prepared to accumulate, enriched uranium", the International Atomic Energy Agency (IAEA) said in a report seen by AFP.

Under the 2015 deal with world powers that puts curbs on Iran's nuclear programme in exchange for sanctions relief, Tehran is only meant to enrich uranium using less efficient IR-1 centrifuges.

The landmark deal has been in jeopardy since May last year when President Donald Trump withdrew the US from it and reimposed sanctions.

The remaining parties to the deal with Iran -- Britain, China, France, Germany and Russia -- have tried to salvage the accord, but Tehran has repeatedly accused Europe of not doing enough.