GENEVA (AP) — 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.

Iran’s refusal to consider suspending enrichment was an indirect slap at the United States, which had sent Undersecretary of State William Burns to the talks in hopes the first-time American presence would encourage Tehran into making concessions.

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.’’

In Washington, a U.S. official was blunter.

‘‘We hope the Iranian people understand that their leaders need to make a choice between cooperation, which would bring benefits to all, and confrontation, which can only led to further isolation,’’ said State Department spokesman Sean McCormack.

In diplomatic terms, ‘‘further isolation’’ is shorthand for economic and political sanctions.

Keyvan Imani, a member of the Iranian delegation cast doubt over the value of talks less then an hour after they started. ‘‘Suspension — there is no chance for that,’’ he told reporters.

Imani also downplayed the presence of Burns — even though the Americans had previously said they would not talk with the Iranians on nuclear issues unless they were ready to stop all enrichment.

‘‘He is (just) a member of the delegation,’’ Imani said.

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.

‘‘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.