However EU foreign policy chief Javier Solana and Iran's chief nuclear negotiator Ali Larijani will meet next Wednesday to discuss the plan, according to foreign ministers of the G8 group of leading nations.
"We expect to hear a clear and substantive Iranian response to these proposals at the planned meeting," the ministers said in a statement in Moscow, where they have been preparing for a July 15-17 summit in St Petersburg.
An official in the US delegation here said foreign ministers from the six countries behind the plan would gather a week later, on July 12, to evaluate Iran's response.
However Iranian Foreign Minister Manouchehr Mottaki said Tehran would not respond before late August.
He said Iran's response "will be clear and substantive," but added that their proposed package contains "questions and ambiguities which must be cleared."
The plan was drawn up by the five permanent UN Security Council members plus Germany, and offers Iran a package of incentives and talks if it suspends nuclear enrichment.
Iran insists its nuclear program is for generating electricity, and that uranium enrichment is needed to provide the fuel.
The European Union and the United States suspect Iran of hiding a military project.