Diplomats from some of the countries taking part - the United States, Britain, France, Germany, Russia and China - said they hoped the Iranian would engage in constructive talks on how to end the long-running standoff over Tehran's nuclear programme, which the West fears is for making weapons.

Iran says its nuclear programme is peaceful and has defied five UN Security Council resolutions demanding it suspend all sensitive nuclear activities.

US officials said Undersecretary of State for Political Affairs William Burns, head of the US delegation, was not actively seeking a one-on-one meeting in Geneva with Iran's chief nuclear negotiator, Saeed Jalili. But they said Burns would not reject one if the opportunity arose.

In Washington, senior Obama administration officials said the United States will not threaten Iran with fresh sanctions at the one-day talks in Geneva but has been actively laying the groundwork to pursue them if necessary.

But the official said the United States has been preparing "a range of areas" in which to pursue sanctions against Iran if Tehran ignores Western entreaties about its nuclear programme.

Iranian Foreign Minister Manouchehr Mottaki, who made a rare visit to Washington on Wednesday to inspect his country's unofficial diplomatic office there, was due to give a news conference at the United Nations in New York October 1.