Washington will send a senior official to meet the Iranian nuclear negotiator this weekend in the most significant American diplomatic contact with Iran since the Islamic Revolution in 1979.

In a major policy shift, William Burns, the third most senior member of the US State Department, will travel to Geneva in Switzerland to attend talks between Tehran and European officials.

The move will not technically end a White House pledge to refuse to negotiate with Tehran until they halt all uranium enrichment because Mr Burns will attend the meeting to listen to the Iranian response to proposals over sanctions rather than enter into negotiations himself.

Last month, Britain, China, France, Germany, Russia and the United States presented Iran with an offer of technological incentives and a pause in increasing sanctions in exchange for suspending uranium enrichment work.

Representatives of those countries will meet Iran’s nuclear negotiator, Saeed Jalili, alongside Javier Solana, the EU’s foreign policy chief.

After a series of bullish statements from Tehran and Washington, this weekend’s meeting signals a significant intensification of diplomatic efforts to resolve the crisis over Iranian nuclear ambitions.

Senator Barack Obama, the next Democratic presidential candidate, has already indicated that he would be open to negotiations with Iran if certain conditions were met. His aides last night backed the change in White House policy that will be formally announced later today.

Iranian and US officials, including Condoleezza Rice, the Secretary of State, have held several meetings in Baghdad to discuss joint concerns about Iraq, but they have not discussed the diplomatic tensions between Washington and Tehran over the nuclear problem.

A State Department official said last night Mr Burns would not participate in further discussions during the proposed cooling off period. "This is a one-time deal," he said.

Under the suggested deal, if Iran halted its nuclear program for six weeks, Western officials would stop adding to the sanctions they have gradually imposed on Tehran for the same period. Full negotiations would then be timetabled.

In June, when Mr Solana travelled to Tehran to present the offer of negotiations the United States refused to join other members of the international coalition in sending a senior official to the meeting.

Sean McCormack, State Department spokesman, said at the time that no American representative would attend unless "Iran suddenly has a change of tune and says that they will meet the demands of the international community, which are expressed in UN Security Council resolutions".

That policy has now been relaxed but the Iranian leadership remains defiant, at least in its public pronouncements.

Ayatollah Ali Khamenei, Iran’s supreme leader, vowed today that Tehran would not accept any threats in negotiations with major world powers over its nuclear drive.

“Iran has decided to take part in negotiations but it will not accept any threat,” state television quoted him as saying.

“Iran’s red line are very clear,” he added, referring to the country’s repeated refusal to accept UN demands to suspend uranium enrichment.

At the Geneva meeting, Mr Jalili is expected to present Iran’s formal response to the package of economic and diplomatic incentives.

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