U.S. Secretary of State John Kerry says if Iran's nuclear program is peaceful, it should have no trouble proving it.
Kerry told reporters Thursday that nuclear talks with Iran later this month can make progress only if Iran is determined to make "real offers and engage in a real dialogue."
He said it is incumbent on Iran to prove it is open to a diplomatic resolution.
Kerry spoke in Washington ahead of his first official meeting with United Nations Secretary General Ban Ki-moon.
The United States and many of its allies suspect Iran is trying to build a nuclear bomb. Iran insists its nuclear program is strictly for peaceful civilian purposes.
Chief U.N. nuclear inspector Herman Nackaerts ended a visit to Iran Thursday with no agreement for new inspections of Iranian nuclear facilities. He gave no details of his talks.
Also Thursday, The Washington Post newspaper reported Iran recently tried to buy tens of thousands of highly specialized magnets for its centrifuges from China. Those ring-shaped magnets are banned for export to Iran by U.N. resolutions. It is unclear if Iran actually got its hands on the magnets.
A European diplomat with access to sensitive intelligence told the Post, on condition of anonymity, that the Iranians "are positioning themselves to make a lot of nuclear progress quickly."
Iran's nuclear chief announced Wednesday that the country was adding thousands of more-advanced, second-generation centrifuges at its nuclear facilities. The upgrade would allow Iran to significantly increase its production of enriched uranium.