Estimates vary, but analysts calculate that anywhere between 1,000-1,700 kilogrammes of low-enriched uranium would be needed to convert it into highly-enriched uranium suitable to make a single atomic bomb.

"Iran has estimated, that between November 18, 2008 and May 31, 2009 ... a total 500 kilogrammes of low-enriched UF6 was produced" at its enrichment plant in Natanz, the International Atomic Energy Agency said in a restricted report, a copy of which was obtained by AFP.

Prior to that, Iran had already amassed 839 kilos of low-enriched UF6.

In all, more than 7,000 uranium-enriching centrifuges were installed at Natanz, up from over 5,000 at the time of the IAEA's last report in February, the watchdog said.

The UN Security Council has ordered Iran to suspend all enrichment related activities, until the IAEA has been able to verify the exact nature of Tehran's controversial nuclear programme.

Western powers fear that Iran wants to build an atomic bomb, but Tehran insists it merely aims to produce civilian nuclear energy.