The offer to Iran over its nuclear programme holds out the prospect that it may be able to enrich uranium in the future, diplomats are quoted as saying.
Iran insists its nuclear programme is solely for the generation of power
"Over the long haul... this Iranian regime can have enrichment at home," a US official told the Washington Post.
US President Bush said Iran's initial reaction to the package of incentives and potential penalties was "positive".
Russian Foreign Minister Sergei Lavrov said Iran might give its full response to the offer by the end of June.
Iran allowed to buy spare parts for civilian aircraft made by US manufacturers
Restrictions lifted on the use of US technology in agriculture
Provision of light water nuclear reactors and enriched fuel
Support for Iranian membership of World Trade Organisation
From Western diplomatic sources
Until now the US and its allies have refused to consider uranium enrichment in Iran, while Tehran says it has a right to do so under international treaties.
The US official, quoted anonymously by the Washington Post, said the possibility of Iran being allowed to enrich uranium depended on it suspending the process for the time-being, and answering "every concern" over fears it is running a nuclear weapons programme.
Separately, a Western diplomat told the AFP news agency: "As part of a very long-term scenario in which everything goes smoothly, enrichment in Iran would be possible.
"But there are a lot of conditions attached. It's too early to talk about Iran enriching. First of all, Iran has to suspend, then we negotiate and everything is on the table."
Iran's chief nuclear negotiator, Ali Larijani, said the package of proposals contained "positive steps".
LIGHT WATER REACTOR
Uses normal water, H20, to cool and moderate uranium core
Uranium used is enriched to include 3-5% fissionable isotope U-235
Nuclear weapons require about 90% U-235
Produces plutonium as by-product, but in insufficient quality for a bomb
Iran has so far refused to accept any deal that relies on it giving up the right to enrich uranium - which it has said is its "inalienable" right.
The BBC's Frances Harrison in Tehran says Iran has turned the nuclear issue into one of national pride, which makes it difficult to back down without being seen to compromise the country's fierce sense of independence.
The incentives package was drawn up by European powers the UK, France and Germany, alongside the US, Russia and China, and delivered to Tehran by the EU foreign policy chief Javier Solana on Tuesday.
It is understood to include permission for Iran to buy spare parts for civilian aircraft made by US manufacturers, and the provision of light water nuclear reactors.
The uranium used to make power in light water reactors needs to be enriched, but this can be done outside the country. The reactors are more difficult than other types to use as a source of plutonium for building nuclear weapons.
28 April: UN nuclear watchdog say Tehran has ignored calls to halt uranium enrichment
Early May: UN debates draft resolution calling for halt to uranium enrichment
Mid-May: EU countries work on proposals to try to induce Iran to curb atomic programme
31 May: US offers to join direct talks with Iran, in major policy shift
1 June: US, Russia, China and three EU states agree on package of incentives and penalties
6 June: EU foreign policy chief presents proposals in Tehran
Other incentives are said to include the lifting of restrictions on the use of US technology in agriculture and support for Iranian membership of the World Trade Organisation.
The US earlier warned Iran a rejection of the proposals could bring UN-imposed penalties.
That would depend on passing a resolution on sanctions at the UN Security Council, where unanimity between the US and Europe on one hand, and Russia and China on the other, has been difficult to achieve.
Russia's Mr Lavrov said on Wednesday that Moscow would only support sanctions against Iran if it violated the nuclear Non-Proliferation Treaty.
Western nations fear Iran is enriching uranium to make nuclear weapons, while Tehran says its nuclear programme is for peaceful energy purposes.