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.