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 US had urged Iran to give the UN nuclear watchdog, the IAEA, "unfettered access" to the uranium enrichment facility within two weeks.
Speaking on CBS television on Sunday, Mr Jones said: "We now have an Iran that is willing to come to the table."
Earlier the head of the IAEA, 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 held talks with the top Iranian nuclear official, Ali Akbar Salehi, and President Mahmoud Ahmadinejad. He is due to leave on Monday
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.
Also on Sunday the US ambassador to the UN, Susan Rice, told NBC television that Iran had a "finite period" to open its nuclear programme to full international inspection.
She declined to set a deadline.
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.