The International Atomic Energy Agency (IAEA) said last week that Iran had revamped its Fordo enrichment plant (FFEP), south of Tehran.

The changes would "significantly increase the rate of production of uranium enriched up to 60 percent", the agency said -- close to the 90 percent needed to make a nuclear weapon.

The rate of production will jump to more than 34 kilogrammes of highly enriched uranium per month, compared to 4.7 kilogrammes previously, it added.

The IAEA called on Iran to implement inspections urgently, while European powers pressed Tehran to "immediately halt its nuclear escalation".

"Iran agreed to the Agency's request to increase the frequency and intensity of the implementation of safeguards measures at FFEP," the IAEA said in a confidential report seen by AFP.

Iran insists on its right to nuclear energy for peaceful purposes and has denied any ambition of developing weapons capability.

But according to the IAEA, it is the only non-nuclear-weapon state enriching uranium to 60 percent purity.

IAEA chief Rafael Grossi told AFP last week that Iran's expansion of enrichment was "a clear message that they are responding to what they feel is pressure".

Iran said last month that it would launch "new and advanced" centrifuges in response to an IAEA resolution that censured Tehran for what the agency called lack of cooperation.

Nuclear tensions between Iran and the West have simmered since Donald Trump withdrew from a landmark 2015 deal with Tehran during his first term as US president, which had exchanged sanctions relief for limits on its nuclear programme.