The International Atmoic Energy Agency says it cannot say for sure whether Iran's nuclear activities are purely peaceful, as Tehran claims, or are designed to develop nuclear weapons.
The IAEA circulated a confidential 32-page report about Iran to diplomats on the governing board.
The agency confirmed that Iran is voluntarily suspending all enrichment-related activities, including the manufacture and import of gas centrifuges that could be used to make nuclear bombs.
The IAEA will start verification of the suspension next week.
The report also details the agency's work and findings in Iran during the past two years. It says inspectors were able to visit several defense-related sites, but are still waiting for permission to visit a complex that Washington says is part of the arms program.
The IAEA admits in the report that its legal authority is limited to pursue verification of possible nuclear weapons-related activity. It says given Iran's past pattern of concealment the investigation has been "time-consuming".
Iran's last-minute co-operation with the U.N. nuclear agency means that it will probably escape referral to the Security Council for possible sanctions.
Shahram Chubin, an Iranian and Director of Research at the Geneva Center for Security Policy, believes a U.S. push for sanctions will probably not receive enough support on the Council, at least for the moment.
"Taking it to the Security Council probably means a divided security Council and probably means that Russia and China, if the Iranians ever want to pull the plug from the negotiations, you could make a case and maybe Iran's friends would say we have to be supportive of the western countries," he said. "But if the Iranians are negotiating in apparently good faith then the idea of imposing sanctions on them will be harder for countries like China and Russia [to accept]."
Both Russia and China are on the IAEA board of governors that meets next week in Vienna to review the Iran case. They are also permanent members of the Security Council, with veto power.