International efforts to halt Iran's uranium enrichment programme have been "overtaken by events", the head of the UN's nuclear agency has said.
Iran says it plans to install 50,000 centrifuges at the Natanz plant
IAEA Director General Mohammed ElBaradei said Tehran now possessed "the knowledge about how to enrich".
In an interview with the New York Times, he said the focus should now be on trying to stop Iran from going to industrial-scale production.
Iran has denied accusations from the West that it seeks nuclear weapons.
Inspectors from the International Atomic Energy Agency revealed that Iran has solved its technical problems and is now capable of making fuel suitable for nuclear reactors.
"From now on, it is simply a question of perfecting that knowledge. People will not like to hear it, but that's a fact," Mr ElBaradei said.
The findings come after a short-notice inspection by the agency at Iran's main nuclear facility at Natanz on Sunday.
"From a proliferation perspective, the fact of the matter is that one of the purposes of suspension - keeping them from getting the knowledge - has been overtaken by events," Mr ElBaradei said.
TIMELINE: IRAN ENRICHMENT
2003: Enrichment programme that had been hidden for 18 years is uncovered by IAEA
Feb 2006: Iran reports to Security Council
5 Feb 2007: Diplomats confirm Iranian claims to have set up more than 300 centrifuges in two cascades
9 April: Iran says it is enriching uranium on an industrial scale. IAEA and Russian officials are sceptical
19 April: IAEA document confirms Iran is running more than 1,300 centrifuges in eight cascades
He added: "The focus now should be to stop them from going to industrial-scale production, to allow us to do a full-court-press inspection and to be sure they remain inside the treaty."
His remarks will serve to increase the pressure on the US, British, French, German and Russian governments, whose collective efforts to halt Iran's nuclear research have so-far failed, says BBC diplomatic correspondent Jonathan Marcus.
Last month the agency revealed the main nuclear plant at Natanz was using about 1,300 centrifuges - machines that spin uranium gas into enriched material.
Until recently, those centrifuges were incapable of running at the speeds necessary to make nuclear fuel.
However, the material being produced by Iran still requires further enrichment before it can be turned into bomb-grade material.
More sanctions threatened
Mr ElBaradei has previously said that Iran would not be able to produce the highly enriched uranium needed for a nuclear bomb as long as it remained under the supervision of IAEA inspectors.
The agency is due to report its findings to the UN Security Council next week.
The UN Security Council has imposed sanctions on Tehran for its failure to scale back its nuclear programme.
Nicholas Burns, the US undersecretary of state for policy, told the New York Times that if Iran does not agree to suspend its activities by the time of next month's G8 meeting, the US would press for a third round of sanctions.
Tehran has insisted that its nuclear programme is for peaceful purposes only.