Amid reports that Iran is continuing to assemble centrifuges that can be used to enrich uranium amid disagreements over the scope of its pledges, the State Department said such activity would be "deeply troubling."
"Failure by Iran to live up to its promise and to adopt a comprehensive and indefinite suspension of all enrichment related and reprocessing activities would be deeply troubling," spokesman Richard Boucher said.
"It would indicate that Iran is neither serious nor genuine about its past promises to the IAEA and to others," he told reporters.
Boucher said Iran had agreed last year to calls from the International Atomic Energy Agency to suspend "all -- all enrichment related and reprocessing activity."
"To begin to rebuild the international community's confidence that Iran has genuinely abandoned its nuclear weapons efforts, the scope of that suspension we believe must be comprehensive," he said.
It "must cover all sensitive aspects of the nuclear fuel cycle and that suspension must continue indefinitely," Boucher added.
His comments came as diplomats in Vienna, where the IAEA is based, said Iran was continuing to assemble the centrifuges despite pledging to suspend uranium enrichment.
The pledge was made as a confidence-building measure in answer to US charges that it is secretly trying to develop nuclear weapons.
Tehran vehemently denies these charges but agreed to the IAEA conditions on enrichment and has signed a key UN treaty protocol allowing surprise inspections of its nuclear facilities.
However, Iran sees the suspension as merely turning off centrifuges used to make highly enriched uranium while the IAEA wants a halt to the whole range of enrichment activities, according to the diplomats.
They say Iran argues that its pledges require them to stop activity at its Natanz nuclear fuel-making plant, but allow it to continue to assemble centrifuges in case it decides to resume making highly-enriched uranium, which can be used both as fuel for reactors or to make the bomb.
IAEA teams are to travel to Iran next week to help prepare a report on Tehran's compliance, which is to be submitted in mid-February ahead of a March meeting of the agency's governing board in Vienna.
Boucher said the United States was looking forward to the February report to establish "whether Iran is meeting, fully, its pledges, including the suspension of enrichment and reprocessing."
"We expect that at the March 8 to 10 meeting of the board of governors that nations will respond appropriately," he said.