WASHINGTON (AFP) - The United States announced Wednesday a senior American diplomat would attend international nuclear talks with Iran in the highest-ranking meeting between the two foes in three decades. In a major policy shift, the White House said Under-Secretary of State William Burns would attend the Saturday talks with Iran on a "one-time" mission to underline Washington's conditions for ending the atomic stalemate. "This will be a one-time US participation to demonstrate that the P5 plus one ... is united in its longstanding principle that Iran, in order to take advantage of an incentives package that is quite generous, has to halt its nuclear enrichment," said White House spokeswoman Dana Perino. She said Burns would go to Geneva "to listen" to Tehran's reply to an incentives offer for freezing uranium enrichment. "We are not there to negotiate," she stressed. The number-three US diplomat's presence at the talks between EU foreign policy chief Javier Solana and Tehran's nuclear negotiator Saeed Jalili will "clarify the consequences" - more sanctions - if Iran rejects the package, she said. Burns's mission to Geneva, a reversal of US President George W. Bush's past policy for grappling with Iran's suspect nuclear program, comes with time fast running out for a solution before Bush's term ends in January 2009. "It's designed to press the advantage, show that we are serious about the diplomatic path, but that there are consequences if Iran doesn't accept the offer. The Iranians have an opportunity here," a senior US administration official told AFP. "The refreshed incentives package, along with new European sanctions and the effects of UNSC sanctions tell us this is the right step." Nonetheless, "It's a one-time deal. Burns will reiterate the suspension condition, but let's be clear, he is there to listen, not to negotiate," the official stressed. Meanwhile, Iran's Supreme leader Ayatollah Ali Khamenei said on Wednesday that his country was ready for negotiations over the nuclear crisis but warned it would not step over any "red lines" in the search for a deal. The supreme leader insisted that Tehran would not be coerced into doing anything against its will. He said that no power could deprive Iran of nuclear technology. "Iran has decided to take part in negotiations but it will not accept any threat," state television quoted Khamenei as saying. He repeated Iran's insistence that it will not suspend uranium enrichment activities. But he also appeared to give his wholehearted backing to the idea of talks. "Our red lines are clear and if other parties respect the Iranian people, the dignity of the Islamic republic and these red lines, our officials will negotiate as long as no one makes any threats against Iran." Israel and the US have both refused to rule out military action against Iran, which in turn has warned that any aggression against its soil will be met with a ferocious response. "We have said clearly that if anyone takes any stupid action against Iran, our response will be decisive," Khamenei warned, saying that even if US President George W Bush ordered a military strike on Iran before leaving office "the Iranian people will pursue him and punish him even if he is not in power." Khamenei also gave strong backing to Ahmadinejad - whose provocative comments on the nuclear crisis have caused controversy even inside Iran - saying it was the President's job to speak on the issue. "The supreme national security council, headed by the President, is responsible on the nuclear question. "What the President and the officials say on the nuclear issue is the fruit of a consensus of all the country's officials," he said.