Iran insists it needs until August to assess an international offer of incentives to get it to halt its controversial nuclear programme.
Iran says its nuclear programme is for peaceful purposes
Tehran was responding to a call by the G8 group of industrialised countries and Russia to give a reply next week.
Iranian Foreign Minister Manouchehr Mottaki said his country was "seriously and carefully" reviewing the package.
But he added that questions still needed to be clarified, and it was reasonable for Iran to take time.
G8 foreign ministers on Thursday urged Iran to reply to the proposals at a meeting on 5 July between EU foreign policy chief Javier Solana and top Iranian nuclear negotiator Ali Larijani.
"We are disappointed in the absence of an official Iranian response to this positive proposal," the ministers said in a statement.
"An agreement of this sort would allow the Iranian people to enjoy the benefits of modern civil nuclear power and would bring Iran many other long-term political and economic advantages."
The ministers were meeting in Moscow ahead of a summit of G8 leaders in mid-July in St Petersburg.
The six major nations involved in negotiating with Tehran - the United States, Britain, France, China, Russia and Germany - will assess the situation just before the summit.
They put together an offer - reportedly including technological help and trade concessions - to try to persuade Tehran to end uranium enrichment.
Iran says its programme is entirely peaceful and is designed to meet its energy needs, but the US and other powers say it is using it to develop nuclear weapons.