US Secretary of State Hillary Clinton has called on Iran to enter next week's nuclear talks in good faith.
The US and its partners will hold talks with the Iranians in Geneva next week.
Mrs Clinton also told the BBC that Iran can enrich uranium for civilian purposes in the future, a rare statement for an American official.
Washington has always been vague when asked whether Iran should one day be allowed to enrich its own uranium for civilian energy.
But Mrs Clinton told the BBC that Iran can enrich uranium for civilian purposes at some future date once it has demonstrated it can do so in a responsible manner and in accordance with Iran's international obligations.
"We told them that they are entitled to the peaceful use of civil nuclear energy," she said.
"But they haven't yet restored the confidence of the international community, to the extent where the international community would feel comfortable allowing them to enrich.
"Iran has to come to the table recognising that they have lost the confidence of even longtime supporters and allies or those who believed them and took them at face value."
While Iran is in theory allowed to enrich uranium as a member of the Nuclear Non-proliferation Treaty, the international community says it cannot enjoy that right while it is in breach of the NPT.
Mrs Clinton's words do not signal a sudden change in policy, but they are one of the clearest indications yet that the US accepts that Iran will one day enrich its own uranium for civilian use.
Israel in contrast insists Iran should never be allowed to do so.
US administrations usually refer to Iran's right to the peaceful use of civilian nuclear energy, but rarely to the right to enrich.
The American secretary of state also said the Iranians came to Geneva talks on Monday with "a much more sober assessment of what isolation means", now that new tough sanctions are in place.
"We know that they're having an effect inside Iran," she said. "We hope that will cause them to have the kind of serious negotiation we're seeking."
The Geneva meeting will be led by the EU's Chief of External Relations, Lady Catherine Ashton.
The last such talks took place a year ago.
Mrs Clinton later spoke at the opening dinner of a security conference, the Manama Dialogue organised by the International Institute for Strategic Studies.
In the same room as her was Iran's Foreign Minister Manouchehr Mottaki.
She urged Iran to restore the confidence of the international community and to make that choice for Iran's people.
Mr Mottaki concentrated on his dinner, while Mrs Clinton was talking.
After the event the US secretary of state was winthin earshot of Mr Mottaki, and greeted him saying "Hello, minister".
The Iranian leader simply mumbled a few indistinct words.