Talks with the five UN Security Council powers plus Germany begin on Thursday in Geneva.
World powers on Wednesday piled pressure on Iran to come clean about its disputed nuclear activities ahead of crucial talks in Geneva.Iran's talks with the five UN Security Council powers plus Germany on Thursday come after Tehran disclosed last week it was building a second uranium enrichment plant, fueling suspicions that it is seeking the capability to build a nuclear bomb.
US Secretary of State Hillary Clinton called on Iran to "comply with its international obligations" by allowing UN inspectors to visit its nuclear facilities and freeze sensitive activities.
"The alternative track," she told reporters at the United Nations in New York, was "greater isolation and international pressure."
A senior US official told reporters in Geneva that the revelation of the new nuclear site in Qom, south of Tehran, had given the six powers a "collective sense of urgency and impatience in this issue."
Iran must meet its obligation to provide the International Atomic Energy Agency, the UN's nuclear watchdog, "full and unfettered access to the Qom site as well as to people and documents connected to that facility," he said.
European Union foreign affairs chief Javier Solana is due to conduct the talks with the top Iranian nuclear negotiator, Saeed Jalili, along with senior officials from Britain, China, France, Germany, Russia and the United States.
The talks will be the first since US President Barack Obama took office in January.
A second senior US official told reporters in Geneva that Iran must reassure negotiators that it will show that its nuclear programme is peaceful.
"We want them to come prepared to focus on the nuclear issue and demonstrate that they are willing to take steps to restore the confidence that's been lost in their peaceful intentions," the official said.
The head of the UN's nuclear watchdog, Mohamed ElBaradei, said that Iran was "on the wrong side of the law" by not declaring its new enrichment plant before last week.
"Iran was supposed to inform us on the day it was decided to construct the facility. They have not done that," IAEA chief Mohamed ElBaradei told Indian TV channel CNN-IBN.
The US demands for cooperation with the IAEA and transparency were echoed in Moscow by Kremlin spokeswoman Natalya Timakovaa, who described the new plant as "an unpleasant surprise for all participants in the process."
But Iran's hardline President Mahmoud Ahmadinejad lashed out at Western demands over the new nuclear site.
"The leaders of these countries made a historic mistake with their comments about the new plant," the state television website quoted Ahmadinejad as saying.
Ahmadinejad said the Geneva talks gave an "exceptional opportunity for the US and a few European countries to correct the way they interact with other world nations."
"The negotiators can definitely adopt any policy that they want, but we will not be harmed," the Fars new agency quoted the president as saying.
Jalili said on Wednesday he was heading for Geneva with a "positive approach" while atomic energy chief Ali Akbar Salehi said Iran was ready to discuss concerns about its new enrichment plant.
However, Salehi also maintained there can be no bargaining about Iran's right to master the civilian nuclear fuel cycle under the nuclear Non-Proliferation Treaty and ruled out a freeze on enrichment.
The UN Security Council has imposed three sets of sanctions against Iran over its refusal to freeze nuclear activities.
"Iran is comprehensively failing to cooperate, it is comprehensively failing to live up to its international commitments," German Chancellor Angela Merkel's spokesman Ulrich Wilhelm told reporters.
A German foreign ministry spokesman said the talks were "an opportunity to establish together whether these words, this assurance of a readiness to talk, will be followed by deeds."
Western countries have maintained their aim for a halt of Iran's uranium enrichment in return for a freeze on sanctions.
"I say to Iran as they face a crucial date this week; join the international community now or face isolation," British Prime Minister Gordon Brown warned Tuesday.
French President Nicolas Sarkozy warned last week that new sanctions would be imposed by December if Iran fails to change its stance.