GENEVA (AP) – The U.S. and five other world powers began high-stakes talks Thursday with Iran to demand a freeze of its nuclear activities, with a senior U.S. official saying Washington is open to rare one-on-one talks with Iranian diplomats.

The EU’s Javier Solana, who is formally heading the one-day negotiations with chief Iranian negotiator Saeed Jalili, was upbeat before the start of the talks in an 18th century villa in Geneva. The U.S. official briefed reporters, speaking on condition of anonymity because of the private nature of the talks.

A bilateral meeting with Iran would reflect Washington’s determination to get results from the meeting.

The fact that the meeting is taking place at all offers some hope, reflecting both sides’ desire to talk, despite a spike in tensions over last week’s revelations by Iran that it had been secretly building a new uranium enrichment plant. But with the U.S. and its allies contemplating new and tighter sanctions on Tehran, there are fears the negotiations may again end in failure.

In addition to the United States and Iran, the countries meeting Thursday include members of the U.N. Security Council and Germany. The U.S. delegation is headed by William Burns, the under secretary of state for political affairs, and Iran’s chief negotiator is Saeed Jalili.

The State Department stressed its hope that the session would open the door to more in-depth dialogue about ways Iran could alleviate concerns that its emerging nuclear program may be secretly developing nuclear weapons.

If Iran is willing to address the nuclear issues, then there likely will be subsequent meetings, State Department spokesman P.J. Crowley said in Washington.

“That process will take some time,” Crowley said. “We’re not going to make a snap judgment on Thursday. We’re going to see how that meeting goes, evaluate the willingness of Iran to engage on these issues.”

Russia’s U.N. Ambassador Vitaly Churkin, asked what Moscow hoped from the talks, said: “To have a start that has a continuation.”

Chinese diplomats have also been urging Iran to negotiate with the six powers, U.N. diplomats said, speaking on condition of anonymity because they were not authorized to speak publicly.

Crowley noted that President Barack Obama has said he intends to take a few months to assess Iran’s position and consult with negotiating partners before deciding what next steps to take.

Diplomats at U.N. headquarters in New York said there has been no discussion of a new sanctions resolution. Several said they wanted to wait for a report from the U.N. nuclear agency on its inspection of Iran’s newly disclosed nuclear facility and to see Tehran’s response to the incentives on the table if it starts negotiations.

Tehran’s acknowledgment that it has kept silent on the plant, which can make both nuclear fuel and fissile warhead cores, has left the Western powers with only modest expectations about the success of the talks.

In Tehran, President Mahmoud Ahmadinejad said the talks will gauge others’ respect for Iran’s rights. “This meeting is a test to measure the extent of sincerity and commitment of some countries to law and justice,” Ahmadinejad said Wednesday, according to official IRNA news agency.

The U.S., Britain, France, Russia and Germany are being represented by senior officials. Only China, which appears most opposed to new U.N. sanctions on Tehran, is sending a relatively low-level representative.

A U.S. government official confirmed to The Associated Press that commercial satellite images taken of the purported nuclear site near Iran’s holy city of Qom are generally accurate.

At least three private sector imagery analysts in the United States have focused on the site because it fits the description given by the U.S. government: it is 20 miles (30 kilometers) from Qom, it is built into a mountainside and it has features consistent with a secret nuclear facility, parts of which the U.S. government says are underground.

Neither the U.S. government nor Iran has officially confirmed the location or the accuracy of the commercial imagery analysis.

At best, Thursday’s talks could start lowering passions over the hidden plant, Iran’s three-year defiance of the U.N. Security Council’s enrichment ban and Western assertions that Tehran is a supporter of terror, and lead to another meeting later this year.

That, in turn, could be the start of a process that could not only end the threat of an Israeli or U.S. strike against Iran’s nuclear facilities as a last resort. It could ultimately lead to an agreement on a limited Iranian uranium enrichment program, but under tight international control meant to banish concerns that it could be turned toward making warhead material.

Such hopes are tenuous. Since the five nations first proposed a set of political and economic concessions to Tehran for a full stop to its enrichment activities three years ago, Iran has expanded the program. It now has more than 8,000 centrifuges set up in its cavernous underground facility at Natanz, with most of them working to churn out fuel-grade enriched uranium.

If the talks fail, the U.S. and its Western allies will renew their push for a fourth set of U.N. Security Council sanctions.

The initial set of sanctions in 2006 focused on banning trade with Iran in materials, equipment, goods and technology that could contribute to Iran’s uranium enrichment program. Iran says its program is intended to provide fuel for civilian power reactors, but the U.S. suspects it could be used to make nuclear weapons. U.N. sanctions against Iran were expanded in March 2007 by banning arms exports from Iran and imposing a freeze on the financial assets of 28 individuals and entities. More sanctions in March 2008 restricted the import by Iran of dual-use technologies, those that can be used for both civilian and military purposes.