Undersecretary of State William Burns will attend the weekend meeting in Geneva between EU foreign policy chief Javier Solana and Tehran's nuclear negotiator Saeed Jalili, a US State Department official said Tuesday.
The United States has no diplomatic relations with Iran and until now has refused to even participate in preliminary discussions with Iranian officials unless Tehran first suspends its nuclear enrichment activities.
Burns, the United States' third-ranking diplomat, would be in Geneva "to receive the Iranian response" to the latest package of incentives for suspension offered by the five permanent members of the UN Security Council plus Germany.
The decision came after Washington insisted for months that it was ready to talk to Iran only if it abandons its enrichment activities, which the West believes is aimed at developing nuclear weapons.
"This is a big break from past policies," David Gergen, a former adviser to US presidents Bill Clinton and Ronald Reagan, told CNN television.
Saying Iran had signalled it might be willing to strike a deal, Gergen said the US administration may sense an opening. "I think they're probably probing that," he said.
Iran's supreme leader Ayatollah Ali Khamenei insisted Wednesday that Tehran would not be coerced into doing anything against its will.
"Iran has decided to take part in negotiations but it will not accept any threat," state television quoted Khamenei as saying.
"Iran's red lines are very clear," he added.
Iran has repeatedly refused to heed UN demands to suspend uranium enrichment, insisting that its operations are exclusively aimed at energy production.
"No power can deprive Iran of nuclear technology," Khamenei said.
A State Department official told the Washington Post that Burns would not negotiate directly or hold separate meetings with the Iranians.
"This is a one-time deal," the official told the Post.
Iran's President Mahmoud Ahmadinejad said on Monday that Jalili and Solana were to discuss a "timetable" for future negotiations.
He also hinted at US-Iran discussions without explanation: "It is possible that in the near future talks in different fields will take place with the United States."
Washington broke off relations with Tehran in 1980 in the wake of the Islamic revolution and the seizure of the US embassy. Ties have remained severed ever since amid increasing acrimony over Iran's disputed nuclear programme.
Last year, US and Iranian ambassadors broke with years of isolation and held talks focused on security in Iraq, with the nuclear issue excluded from the agenda.
There have also been reports of a possible diplomatic initiative from Washington with President George W. Bush's administration considering opening a US interests section in Tehran.
Last month the P5 plus 1 -- Britain, China, France, Germany, Russia and the United States -- presented Iran with an offer of technological incentives in exchange for suspending sensitive uranium enrichment work.
European diplomats say Solana is proposing to Tehran that world powers would refrain from new sanctions provided Iran did not start operating any additional centrifuges to enrich uranium.
Iran has countered with a proposal which it says will go toward solving some of the major security problems of the world, and which diplomats have described as complex.
Officials hope the Geneva meeting will permit Iran and the major powers to move from "talks about talks" to a phase of "pre-negotiation", in which Iran would stop bringing new centrifuges on line and sanction action would stop.
© 2008 Agence France Presse