Iran insists its nuclear programme is for peaceful purposes
US Secretary of State Hillary Clinton has said Iran must answer "head on" concerns about its nuclear programme at talks with world powers on 1 October.
Mrs Clinton said the issue "cannot be ignored" and was the key reason why the US agreed to take part in the talks.
Tehran last week offered "comprehensive" talks, but did not mention its nuclear programme.
The West fears that Iran is secretly developing nuclear weapons - a claim denied by Tehran.
Iran insists its programme is for civilian purposes only.
"We have made clear to the Iranians that any talks we participate in must address the nuclear issue head on," Mrs Clinton said in Washington.
Hillary Clinton said the talks were a fulfilment of Obama's pledge
"Iran says it has a number of issues it wishes to discuss with us but what we are concerned about is discussing with them the questions surrounding their nuclear programme and ambitions.
"We will wait to see how Iran responds in that face-to-face venue," America's top diplomat said.
She added the talks marked a "fulfilment" of US President Barack Obama's pledge to engage with Iran.
The meeting between Iran and the so-called P5+1 Group - the five permanent members of the UN Security Council - the UK, China, France, Russia and the US - plus Germany - are due to start at an as yet undecided venue.
The P5+1 Group has a longstanding offer on the table of diplomatic incentives in return for the suspension of Iran's uranium enrichment.
Iran has always defended its right to continue its nuclear programme.
Last week, Tehran offered to "embark on comprehensive, all-encompassing and constructive negotiations", but ignored the nuclear issue.
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