"We urge Iran to suspend its uranium enrichment and reprocessing activities or face further implementation of the existing United Nations Security Council sanctions and the possibility of new sanctions," said spokesman Gordon Johndroe.
The International Atomic Energy Agency charged in a restricted report, a copy of which was seen by AFP, that the Islamic republic has not frozen uranium enrichment activities, which can be a key step towards nuclear weapons.
"This report shows once again that Iran is refusing to cooperate with the international community. The Iranian regime's continued defiance only further isolates the Iranian people," said Johndroe.
The IAEA "regrettably has not been able to make any substantive progress on the alleged studies and other associated key remaining issues which remain of serious concern," the agency said in the report.
"On this particular issue, we've arrived at a gridlock," a senior official close to the IAEA told reporters, speaking on condition of anonymity.
The so-called "alleged studies" suggest Iran may have been trying to develop a nuclear warhead, convert uranium and test high explosives and a missile re-entry vehicle.
But Iran has so far done little to disprove the allegations, merely dismissing the documentation used to back them up as "forged" and "fabricated," the International Atomic Energy Agency said.
The report is to be discussed by the IAEA's board of governors next week.
IAEA Director General Mohamed ElBaradei "urges Iran to implement all measures required to build confidence in the exclusively peaceful nature of its nuclear program at the earliest possible date," the report concluded.
