Mohamed ElBaradei: "I see that we are at a critical moment"
The head of the UN nuclear watchdog has said relations with Iran are shifting from confrontation to co-operation.
Mohamed ElBaradei, who is visiting Tehran, said this was a "critical moment" and urged Iran to be as transparent as possible.
The head of the International Atomic Energy Agency also announced inspectors would visit Tehran's newly revealed uranium enrichment plant on 25 October.
Iran denies concerns it is developing a nuclear weapons capability.
The US had urged the Iranian government to give the IAEA "unfettered access" within two weeks to the site disclosed last month. The uranium enrichment facility is located near the city of Qom.
Speaking after talks with the top Iranian nuclear official, Ali Akbar Salehi, Mr Elbaradei said the talks had been successful, and that inspectors would ensure that the Qom facility was for "peaceful purposes".
IRAN'S NUCLEAR SITES
Iran insists that all its nuclear facilities are for energy, not military purposes
Bushehr: Nuclear power plant
Isfahan: Uranium conversion plant
Natanz: Uranium enrichment plant, 4,592 working centrifuges, with 3,716 more installed
Second enrichment plant: Existence revealed to IAEA in Sept 2009. Separate reports say it is near Qom, and not yet operational
He added: "I see that we are shifting gears from confrontation into transparency and co-operation. I continue, of course, to call on Iran to be as transparent as possible."
Mr ElBaradei's visit comes as the New York Times quoted an internal IAEA report as saying Iran could have the know-how to produce a workable nuclear bomb.
The confidential report, excerpts of which have also been published on the website of the Institute for Science and International Security (ISIS), emphasises that its conclusions are tentative and unconfirmed.
"The agency... assesses that Iran has sufficient information to be able to design and produce a workable implosion nuclear device based upon highly enriched uranium as the fission fuel," ISIS quoted the report as saying.
The New York Times said this went well beyond the public positions taken by the US and other countries on the possibility of Iran creating a bomb, but Mr ElBaradei had raised doubts about the report's reliability.
The IAEA chief is in Tehran until Monday.
He also said there had been a "positive response" from several suppliers to Iranian requests for its enriched uranium to be reprocessed abroad before being returned for use as medical isotopes.
Iran says the enrichment plant at Qom is not operational yet
Iranian, Russian, French and US officials would meet in Vienna on 19 October to "hammer out an agreement" on this, he added.
Mr Elbaradei later met President Mahmoud Ahmadinejad, but is not expected to visit any nuclear facility.
Tehran insists it has the right to develop nuclear energy, but the revelation of the second enrichment facility has heightened fears among Western governments that it is trying to develop nuclear weapons.
The visit comes after talks in Geneva earlier this week between Iran and six major powers - the five permanent members of the Security Council plus Germany. They were the first such talks since July 2008.
Officials said they marked "engagement" on the part of Iran after the country agreed to co-operate "fully and immediately" on the second enrichment facility.
The two sides also agreed to hold further talks in October.
US President Barack Obama underlined on Thursday that the US expected Tehran to take "constructive actions" to convince the world that it was not trying to build a nuclear weapon.
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