A U.S. decision to bend policy and sit down with Iran at nuclear talks fizzled Saturday, with Iran stonewalling Washington and five other world powers on their call to freeze uranium enrichment.

In response, the six gave Iran two weeks to respond to their demand, setting the stage for a new round of U.N. sanctions.

Officials and diplomats refused to characterize the timeframe as an ultimatum, but it appeared clear that Iran now has a de-facto deadline to show flexibility.

EU envoy Javier Solana said that Iran still has to answer a request made on behalf of the five permanent U.N. Security Council members plus Germany to "refrain from any new nuclear activity."

"We have not gotten all the answers to the questions," Solana told reporters. He said the two-week timeframe was meant to give Iran the space to come up with "the answers that will allow us to continue."

He said the contact might be on the phone, or through aides, reflecting the lack of success of Saturday's highest-level discussions.

A Western diplomat familiar with the substance of the talks was blunter.

Unless Iran shows some flexibility on suspension, he said, the six powers will "look to strengthen measures in the U.N. and the European Union" _ shorthand for economic and political sanctions. He demanded anonymity because his information on the outcome of the closed meeting was confidential, along with the planned response if Iran remains defiant.

Chief Iranian negotiator Saeed Jalili evaded the issue of suspension, demanded as part of the six-power proposal that carries a commitment of no new U.N. sanctions in exchange for an Iranian pledge to stop expanding its enrichment program.

Instead he spoke in generalities about the need for cooperation that reflected Iran's reluctance to focus on the six-power offer.

"Iran is calling on the Western powers to resume the dialogue," he said.

Iran already is under three sets of U.N. sanctions for its refusal to suspend enrichment, which can generate both nuclear fuel and the fissile material at the core of nuclear warheads. While Tehran says it has a right to enrich for peaceful purposes, the sanctions reflect international concern that it might use its program to make weapons.

The offer delivered to Iranian officials last month by Solana envisions a six-week commitment from Iran to stop expanding enrichment and from their interlocutors to agree to a moratorium on new sanctions for up to six weeks.

That is meant to create the framework for formal negotiations which the six nations hope would secure Iran's commitment to an indefinite ban on enrichment.

Iran's apparent stonewalling was an indirect affront to the U.S., which had sent Undersecretary of State William Burns to the talks in hopes the first-time American presence would encourage concessions.

Recent Iranian statements had suggested the country is looking to improve ties with the United States, with officials speaking positively of deliberations by the Bush administration to open an interests section _ an informal diplomatic presence _ in Tehran after closing its embassy decades ago.

Burns' decision to attend the Geneva talks showed that Washington was willing to accept something less than fully dismantling the program as it had always demanded _ at least as a first step.

U.S officials had insisted Burns was at the table to listen only, describing his presence as a one-time occasion. But State Department spokesman Sean McCormack said otherwise.

Burns delivered "a clear simple message" when it was his turn to speak, McCormack told reporters in Washington.

He cited Burns as telling the his Tehran counterpart: "Iran must suspend uranium enrichment to have negotiations involving the United States."

Iran needed now to "make a choice between cooperation, which would bring benefits to all, and confrontation, which can only led to further isolation," McCormack said.

Iran and the United States broke off diplomatic relations after the 1979 Islamic Revolution and the hostage crisis at the U.S. Embassy in Tehran. Official contacts between the two countries are extremely rare.

Keyvan Imani, a member of the Iranian delegation, said Tehran had not yet received a proposal from the U.S. on a representation but would "study it positively" if it did.

He downplayed the presence of Burns _ even though the Americans had previously said they would not talk with the Iranians on nuclear issues unless Tehran was ready to stop all enrichment activity. "He is (just) a member of the delegation" of the six countries engaging Iran on the nuclear issue, Imani said.

He also cast doubt over the value of talks less then an hour after they started. "Suspension _ there is no chance for that," he told reporters gathered in the courtyard of Geneva's ornate City Hall, the venue of the negotiations.

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Associated Press Writer Bradley S. Klapper contributed to this report from Geneva.