US Secretary of State Colin Powell has repeated his demand for Iran to be referred to the UN Security Council for allegedly trying to make nuclear arms.
Iran denies it wants to build nuclear weapons
Mr Powell was responding to reports that Iran had offered to stop producing centrifuges used to enrich uranium.
He said he wanted Tehran to take steps to stop producing material that could lead to nuclear weapons.
Iran's reported concession comes days before its case is due before a meeting of the UN's nuclear watchdog.
Mr Powell said the board of the International Atomic Energy Agency (IAEA) should refer Iran to the Security Council for possible sanctions.
He said Iran has repeatedly failed to comply with its international obligations under the Nuclear Non-Proliferation Treaty.
"We thought it should have been referred to the Security Council last November but colleagues of mine, particularly the European Union Three [Britain, France and Germany] wanted to engage Iran and find a solution," he said
"Unfortunately, I don't think the response from Iran has been very positive or constructive either to what the EU Three commitments were or to the IAEA," he added.
The US accuses Iran of working towards building nuclear weapons, but that has not been proved.
Enriched uranium can be used to generate power or for nuclear warheads, depending on the level of enrichment.
Tehran says its nuclear programme is simply for electricity.
It had previously agreed to temporarily suspend building, assembling and testing uranium enrichment centrifuges.
However, Tehran announced it was resuming enrichment work after an IAEA resolution in June criticised it for inadequate co-operation with UN inspectors.