Mohamed ElBaradei: "I see that we are at a critical moment"
The US and the UN have given an upbeat assessment of the possibility of settling the nuclear dispute with Iran.
US National Security Adviser Jim Jones said Tehran was now "willing to come to the table", following talks this week between Iran and major powers.
The head of the UN's nuclear agency, who is in Tehran, said there had been a "gear shift" towards co-operation.
He announced that inspectors would visit a newly revealed nuclear site in the Iranian city of Qom, on 25 October.
The BBC's Richard Lister in Washington says the US administration is still a long way from taking Iranian co-operation on the nuclear issue for granted, but it does see some reasons for optimism.
Asked about Iran's offer to allow inspection of the Qom uranium enrichment facility and its agreement in principle to ship nuclear materials to Russia for re-processing, Mr Jones said these were "very significant" moves.
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
"We now have an Iran that is willing to come to the table," he told CBS television on Sunday.
Earlier the head of the UN's International Atomic Energy Agency, Mohamed ElBaradei, said in Tehran that inspectors would ensure that the Qom facility was for "peaceful purposes".
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," 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.
Mr Jones told CBS that whether Tehran was in a position to do so remained a matter of "conjecture".
However on Sunday the US ambassador to the UN, Susan Rice, told NBC television that Iran had a "finite period" to come to a settlement on its nuclear programme.
"We're not interested in talking for talking sake," she said.
Iran says the enrichment plant at Qom is not operational yet
But she declined to set a deadline for Tehran to allow full international inspections.
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.
Iran denies any attempt to develop a military nuclear capability.
The UN Security Council has demanded a halt to uranium enrichment by Iran.
The talks in Geneva earlier this week between Iran and six major powers - the five permanent members of the Security Council plus Germany - were the first since July 2008.
Officials said the talks marked "engagement" on the part of Tehran after the country agreed to co-operate "fully and immediately" on opening the second enrichment facility to inspectors.
The two sides also agreed to hold further talks in October.
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