Speaking to reporters in Vienna, Rice repeated Washington's offer to join multiparty talks on trade, security and technological benefits for Iran if the Islamic state stopped its enrichment activities.

"I think it's time for Iran to change its tactics," Rice said.

"The international community is united on what Iran should do and that is to suspend, to demonstrate that it is in fact not seeking a nuclear weapon under cover of civil nuclear power."

Her comments came hours before Iran's top negotiator Ali Larijani and EU foreign policy chief Javier Solana were scheduled to begin a fresh round of talks in Madrid aimed at seeking a way out of the increasingly tense nuclear standoff.

It will be their second meeting in just over a month after a fruitless head-to-head in Turkey in late April.

It's also their first encounter since a 60-day time limit set by the United Nations for Iran to stop enriching uranium, a process which makes reactor fuel but also atom bomb material, expired last week.

The dispute over Iran's persistent defiance of UN demands has intensified in recent days, with stepped up rhetoric on both sides of the debate.

Foreign ministers from the Group of Eight most industrialised nations said Wednesday that they were prepared to back "appropriate measures" if Iran failed to change its stance.

Oil-rich Iran denies it is seeking nuclear weapons, saying it wants only to produce energy for a growing population whose fossil fuels will eventually run out.

The US is leading calls by Western powers for existing sanctions on Iran to be tightened. The UN Security Council first imposed sanctions on Iran in December for rejecting its demands, and then modestly increased them in March.