GENTHOD, Switzerland — The U.S. and five other world powers began high-stakes talks today with Iran to demand a freeze of its nuclear activities, with the United States 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 the ornate villa in Genthod, northeast of Geneva.

Solana opened the discussions, followed by chief U.S. delegate William Burns, with the six powers sitting on one side of a large oval table and the three-man Iranian team on the other, officials inside the meeting reported. They spoke on condition of anonymity because the talks were closed.

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.

U.S. willingness to talk one-on-one with Tehran during breaks in the meeting also showed Washington's determination to keep the dialogue going. At the same time, U.S. has signaled it is already contemplating new and tighter sanctions on Tehran, reflecting expectations that the talks may end in failure.

While the West fears that Iran's nuclear program aims to make a bomb, Iran insists the program is strictly for peaceful use and has refused to negotiate any limits on it.

Iran is bringing a broad range of geopolitical issues to the table, while the six powers — the permanent U.N. Security Council members plus Germany — are seeking to soften Iran's resistance to freezing its uranium enrichment program. The process can make both nuclear fuel and fissile warhead material.

U.S. spokesman Robert Wood said the six would also raise concerns about Iran's recent revelation it is building a second enrichment plant, alongside one that is under supervision by the International Atomic Energy Agency to make sure it makes only fuel and not weapons-grade uranium.