The five permanent members of the UN Security Council and Germany are due to hold telephone talks on the next step to take on Iran's nuclear programme.
Iran contacted the EU's foreign policy chief on Tuesday, but did not give a full answer to an incentives offer in return for halting uranium enrichment.
In a letter, Iran said it was ready to give a clear response but in return wanted a clear answer to its questions.
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The telephone conference between the six nations - China, France, Russia, the UK and US, along with Germany - was due to take place at 1115 GMT on Wednesday.
On Monday, the US and UK said Iran faced further sanctions if it did not respond positively to the proposals.
The move follows "inconclusive" talks between the EU and Tehran.
Freeze demanded
The group of six world powers tabled the latest proposal to Iran in June in an effort to persuade it to suspend uranium enrichment as an initial step in starting talks on a longer-term deal.
EU foreign policy chief Javier Solana had given Iran's chief nuclear negotiator until last Saturday to respond to an offer not to impose further economic sanctions in return for Iran freezing its uranium enrichment programme.
The Iranian government insists its nuclear programme is for entirely peaceful purposes, while the US and its allies believe it could be used to develop nuclear weapons.
In its letter to the six nations, obtained by the AFP news agency, Iran said it had considered the proposals "with good will" and a constructive approach to continued negotiations.
"Now the Islamic Republic of Iran is ready to provide a 'clear response' to your proposal at the earliest opportunity while simultaneously expecting to receive your 'clear response' to our questions and ambiguities as well," the letter continues.
"Undoubtedly such mutual clarification can pave the way for a speedy and transparent negotiating process with a bright prospect and provide grounds for co-operation.
"The second phase in negotiations can commence as early as possible if there is such willingness on your side."
The New York Times, quoting US and European diplomats, said the US administration considered the letter "unacceptable" and would seek new sanctions against Iran at the UN Security Council.
BBC diplomatic correspondent Jonathan Marcus says that Iran's response matters because it will determine how far Russia and China are willing to support additional UN Security Council sanctions.
He says the Iranians have clearly made a judgement that despite all the talk of potential air strikes against its nuclear facilities, the US presidential campaign, Israel's political uncertainty and high oil prices mean that a crisis is not imminent.
(BBC)
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