GENEVA - The six main 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 a new secret Iranian uranium enrichment plant was disclosed. Iran also carried out tests of missiles this week, again rattling international nerves.

The US envoy at the talks, William Burns, also held a one-on-one meeting with Iran's top nuclear negotiator Saeed Jalili, a rare official encounter between representatives of the arch-foes, a US spokesman said.

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 State Department deputy spokesman Robert Wood said the talks with the UN Security Council's five permanent members and Germany provide Iran an opportunity to come clean about its nuclear ambitions.

But he warned that US patience has its limits.

"This is the first time that we've agreed to sit down with Iran as a full member of the P5+1 discussions," he told journalists.

"We're willing to engage in this process but we're not going to do it forever. There's going to come a point when we're not going to engage," he said.

US Secretary of State Hillary Clinton warned Iran on the eve of the talks that it risks "greater isolation and international pressure" if it refuses to give 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 nuclear suspicions by December.

Iran denies that it is seeking a bomb. But 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.

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 and the international envoys are the first since US President Barack Obama took office in January.

The Iranians began the meeting by reiterating that the nuclear standoff was part of a wider range of issues that need to be addressed, "but at least they were talking nuclear towards the end of the morning," a Western diplomat said.

The United States wants Iran to respond to an international offer to suspend sanctions in exchange for Iran halting nuclear enrichment. The revelation of the new site has added a sense of urgency, a US 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. The IAEA asked Iran for access to the site "as soon as possible," Iranian state television said.

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

He also proposed a framework for discussions with the six powers, including a "summit of leaders to talk about all proposals" of common interest, including nuclear proliferation.