The U.S. is worried Iran is making deliberate attempts to preserve a nuclear weapons option.
Glyn Davies, Washington's chief envoy to the International Atomic Energy Agency, said Tehran is either close to or already in possession of enough low-enriched uranium to make a bomb, should the material be further enriched.
"This ongoing enrichment activity ... moves Iran closer to a dangerous and destabilizing possible breakout capacity," Davies told the agency's 35-nation board of governors.
"Taken in connection with Iran's refusal to engage with the IAEA regarding its past nuclear warhead-related work, we have serious concerns that Iran is deliberately attempting, at a minimum, to preserve a nuclear weapons option," Davies said.
But Iran remains adamant that its intentions are peaceful and that the goal of the program is to generate electricity.
"Regarding Iran's nuclear issue, if there are any questions or ambiguities, we are well prepared to remove ambiguities in the context of the IAEA," Tehran's envoy Ali Asghar Soltanieh told reporters.
Davies said the U.S. is open to working with Iran to resolve the matter.
"This is a fresh, new opportunity for Iran to turn the page, come back to the negotiating table and prove that it is a responsible, trustworthy member of the international community," Davies said.
Iran has until the end of September to suspend enrichment activities - otherwise it will face more severe sanctions.