Iran's chief nuclear negotiator, Ali Larijani, has said his country is ready to start "serious talks" with six world powers on Wednesday.
Iran says its nuclear programme is for peaceful purposes
Iran has submitted a written response to the demand by the five permanent UN Security Council members plus Germany to suspend uranium enrichment.
It has until the end of August to suspend enrichment in exchange for incentives, or risk possible sanctions.
Iran denies Western claims that it is developing a nuclear weapons programme.
Tehran has always maintained that it has a right to civilian nuclear technology, and Supreme leader Ali Khamenei made it clear on Monday that Iran would reject international pressure.
Although Mr Larijani has spoken of "serious talks", what the Security Council needs to know is whether Iran is willing to suspend uranium enrichment by 31 August or not, says the BBC News website's world affairs correspondent, Paul Reynolds.
If it is not, or gives no clear response on this, the US and its allies will take it as a "no" and will press for sanctions, though these would need a separate council decision, our correspondent adds.
The US, UK, Russia, China, France and Germany have offered Iran a package of incentives - including the offer of help with civilian nuclear technology. The details of Tehran's written response have not been made public.
The European Union's foreign policy chief, Javier Solana, said the Iranian document was "extensive" and required "a detailed and careful analysis."
The US ambassador to the UN, John Bolton, said the choice facing Iran was clear.
"They can either take up the very generous offer that the five permanent members and Germany have extended to them, and if they do there's a possibility of a different relationship with the United States and others.
"But if they don't we've also made it clear that their unwillingness to give up their pursuit of nuclear weapons will result in our efforts in the Security Council to obtain economic sanctions against them."
Iranian officials had previously said the response would address ambiguities over its right to nuclear technology.
Enriched uranium is used as fuel for nuclear reactors, but highly enriched uranium can also be used to make nuclear bombs.
Iran points out that as a signatory to the nuclear non-proliferation treaty (NPT) it is entitled to a nuclear power programme and says it has broken no rule.
But the Western powers accuse Iran of concealing an enrichment programme, and Washington has refused to rule out military action.