Six world powers on Thursday started crunch talks with Iran seeking to pressure Tehran to prove that its nuclear programme is peaceful.

Iran's top nuclear negotiator met officials from the United States, Russia, China, Britain, France and Germany in a villa overlooking Lake Geneva only a week after the disclosure of a second Iranian uranium enrichment plant. Iran has also carried out missile tests this week which it says could hit Israel.

Amid widening Western concerns about whether Iran is trying to build a nuclear bomb, the international powers have urged Iran to give the International Atomic Energy Agency access to the previously secret nuclear site near the holy city of Qom.

US Secretary of State Hillary Clinton warned Iran on the eve of the talks that it risks "greater isolation and international pressure" if it fails to give UN inspectors access to nuclear facilities and freeze sensitive activities.

French Defence Minister Herve Morin said his country would press for new sanctions against Iran if it fails to clear up suspicions about its nuclear programme by December. Iran denies that it is seeking a bomb.

The UN Security Council has already imposed three sets of sanctions on Iran for refusing to end its uranium enrichment.

"Iran is in total contradiction with its international commitments and every day shows that it is pursuing its nuclear military programme," Morin told Le Figaro newspaper.

But French Foreign Minister Bernard Kouchner said he hoped a breakthrough could be reached in Geneva to avoid sanctions.

"I am not a fanatic of sanctions against the people (of Iran). Sometimes they are useful but we are not talking about the sanctions in Geneva so far," he said on a visit to Moscow.

Russia and China have been reluctant to impose more sanctions on Iran over its nuclear programme.

Israel's vice prime minister Silvan Shalom dismissed the talks as a "waste of time," saying Iran would never give up its alleged bid to acquire nuclear weapons. He called for "real" sanctions.

The talks between Iran's top nuclear negotiator Saeed Jalili and senior officials from the five UN Security Council members, plus Germany, are the first since US President Barack Obama took office in January.

A spokesman for the US mission confirmed that Washington's representative was "present at the table" at the villa in the lakeside village of Genthod overlooking the Alps on the outskirts of Geneva.

The "starting point" for the negotiations will be the six powers' offer to suspend sanctions against Iran in exchange for Tehran to freeze its nuclear enrichment, a senior US official in Geneva told reporters before the talks.

The revelation of the new site has added a "sense of urgency and impatience" among world powers, the official said.

IAEA chief Mohamed ElBaradei said Iran was "on the wrong side of the law" by not declaring the new plant to his agency before last week.

Iran's atomic chief, Ali Akbar Salehi, said this week that his country would soon give a timetable to inspect the second site.

The United States, which broke diplomatic relations with Iran 30 years ago, is prepared to enter one-on-one dialogue with Iran at the talks, a senior US official said on condition of anonymity.

The official said in Washington that it was up to chief US negotiator William Burns to decide whether the process could be helped by talking to the Iranian side directly.

"It will also provide for an opportunity, if it's useful in the talks, for there to be bilateral conversations between members of the P5+1 group and the Iranian group in Geneva," the official added.

Iranian President Mahmoud Ahmadinejad signalled for the first time Wednesday a willingness to discuss specifics about its enrichment operations at the talks, saying Tehran could allow a third party to enrich uranium for a reactor.

But the hardline leader remained defiant, insisting that Iran would not be "harmed" whatever the outcome of the negotiations.