The United States has rejected an Iranian offer to consider ceding more authority to nuclear inspectors in return for access to nuclear technology.
"That's a nonstarter ... It's not a bargaining point. It's a point of living up to international standards that everybody else feels comfortable living up to," State Department spokesman Mr Richard Boucher said.
The United States and the United Nations nuclear watchdog, the International Atomic Energy Agency (IAEA), are pressing Iran to sign the Additional Protocol, which gives the agency the right to impose a strict nuclear inspections regime.
A spokesman for Iran's Atomic Energy Organisation said Tehran might agree to sign it, but reiterated Iran's demands for access to nuclear technology in exchange.
"We have not yet decided about signing the Additional Protocol, but we are studying it with a positive view," spokesman Mr Khalil Mousavi said.
Iran's nuclear programs are the centre of attention at a meeting in Vienna of the IAEA board of governors, which the United States expects to back its position on Iran. Washington says Iran plans to make nuclear weapons but Iran denies the calim.
"We're looking for the board to make clear its concern, to call on Iran to fix the problems, to call on Iran to sign the Additional Protocol and support the efforts of the IAEA to find out more information and get compliance," Mr Boucher said.