Iran has accelerated its enrichment of uranium to near weapons grade, the UN atomic watchdog said in a report on Tuesday, Reuters reported.
The move has increased tension with the West as both sides seek to resume talks on reviving the 2015 nuclear deal between Iran and global powers.
Iran is refining uranium to 60 per cent fissile purity from 20 per cent in April, in response to an explosion and power cut at its Natanz site, which damaged production at the main underground enrichment plant.
Iran has blamed the attack on Israel. Weapons grade is about 90 per cent purity.
In May the International Atomic Energy Agency reported that Iran was using one cascade, or cluster, of advanced centrifuges to enrich uranium up to 60 per cent at its above-ground pilot plant at Natanz.
The IAEA told member states on Tuesday that Tehran had added a second cascade, for a total of 317 centrifuges, to enrich uranium to the new level, the IAEA said.
The move is the latest of many by Iran breaching the restrictions imposed in the 2015 nuclear deal, which capped the purity to which Tehran can refine uranium at 3.67 per cent.
The US and its European allies have warned that such moves threaten talks on reviving the deal, which have been suspended.
On Monday, the IAEA said Iran had made progress in its work on enriched uranium metal despite objections by western powers that there was no civilian use for such work.
Uranium metal can be used to make the core of a nuclear bomb but Iran says its aims are peaceful and it is developing reactor fuel.