BBC: A senior United States official is taking part for the first time in international talks with Iran over its controversial nuclear programme.
The official, William Burns, is joining envoys from the EU and permanent members of the UN security council.
They are expected to discuss with the country's top nuclear negotiator, Saeed Jalili, incentives for Iran to suspend uranium enrichment.
Mr Burns' attendance is being seen as a major shift in US policy.
The US and Iran have had no diplomatic relations since the 1979 Iranian Revolution and the taking of hostages at the US embassy in Tehran.
Formal contact between the two countries have been extremely limited, though last year they met at ambassadorial level to discuss security in Iraq. US Attends Historic Iran Meeting >>> | July 19, 2008
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