The United States on Sunday condemned Iran for persisting with what Washington regards as an atomic bomb-making programme.
"Iran needs to come clean and fully co-operate with its international obligations," White House spokesman Scott McClellan told reporters travelling with US President George W. Bush to a Nato summit in Istanbul.
"Iran's continued failure to comply with the International Atomic Energy Agency and continued failure to stop all enrichment-related reprocessing activities only reinforce the concern we have expressed," McClellan said.
Iran said on Sunday it would resist international pressure to reverse its decision to produce parts for centrifuges that enrich uranium, reneging on a pledge to suspend all enrichment activities.
If enriched to a low level, uranium can be used as fuel for electricity-generating reactors such as the one Iran is building on its south coast. But if enriched further, to weapons-grade, it can be deployed in warheads.
Washington has pushed its Western allies to take a tougher line on Iran but Britain, Germany and France have resisted, preferring to try to persuade Tehran that it is in its interests to come clean on nuclear activities.
The US official said Iran's defiance would help persuade other countries to consider referring the issue to the United Nations, which could impose economic sanctions.
"We have expressed concern within the IAEA about the need to consider sending this matter to the Security Council of the United Nations and I think this latest move may only serve to convince others of the need to seriously consider that step," McClellan said.
The United States, the European Union and the IAEA condemned Iran on Saturday for resuming centrifuge part production and urged Iran to rethink its decision.
Iran insists its ambitions are entirely peaceful and has said it has no immediate plans to pump uranium hexafluoride gas into spinning centrifuges to start the enrichment process.