The US said Mr Burns would not negotiate but was there to listen
A top US official is to attend talks aimed at persuading Iran to halt its nuclear enrichment programme.
Under Secretary of State William Burns will travel to Switzerland with the EU foreign envoy Javier Solana to receive Iran's response to a UN offer.
The US said Mr Burns would not hold separate talks with Iranian negotiator Saeed Jalili, and was there to listen.
A BBC correspondent says this is a first under the Bush administration and represents a significant policy shift.
Iran denies any nuclear weapons plans, but is defying UN Security Council demands to halt uranium enrichment.
In the past, the Bush administration has insisted that no talks will be held with Iran until it suspends its uranium enrichment programme.
Now, in a clear shift in policy, it seems a meeting is going to take place without that precondition being met, says the BBC's Jonathan Beale in Washington.
The Bush administration says Mr Burns's presence is designed to demonstrate the West's unity and to reiterate that the terms of negotiations remain the same - namely that Iran must halt its uranium enrichment programme for further talks to take place.
How significant the move is will become clearer after the talks in Geneva on Saturday, our correspondent adds.
There are growing signs that Iran and America are taking significant steps towards a negotiated solution to the nuclear crisis, says the BBC's Tehran correspondent, Jon Leyne.
Iranian President Mahmoud Ahmadinejad said earlier this week that he was interested in direct talks with the US, and in opening of a US diplomatic mission in Tehran for the first time since the revolution.
Iran will probably soon accept a Western proposal to freeze its nuclear programme at its current level, in return for a freeze on new sanctions, our correspondent says.
The key issue of suspending uranium enrichment is more difficult, but is under active consideration, our correspondent says.
A State Department official told the BBC that Mr Burns - the third-most senior US diplomat - would "reiterate that our terms of negotiation remain the same - that Iran must suspend its enrichment and reprocessing".
He went on: "This will be a one-time participation by Ambassador Burns."
The US is among six world powers which have offered Iran negotiations on a package of incentives, including direct contact and dialogue, if Tehran suspends uranium enrichment, a process the West fears could be used to make nuclear weapons.
But it has not previously sent US diplomats to sit down face-to-face with Iranian negotiators to discuss the issue.
The purpose of the talks is to find out how Iran will respond to the West's offer of economic incentives if Tehran suspends its uranium enrichment programme.
The package brought to Tehran by Mr Solana last month includes a series of proposals designed to help Iran develop a civilian nuclear programme.
The meeting comes at a time of increased tension between Iran and the US, particularly following Iranian missile tests last week that prompted the US to warn that it would defend its interests and its allies in the region.
Formal contacts between the US and Iran are very rare, though the two countries held three rounds of talks in 2007 over Iraq.
The two nations have not had diplomatic relations since the 1979 Iranian Revolution and the taking of hostages at the US embassy in Tehran.