Iran has brought forward its deadline for European states to submit their proposals to resolve the dispute over Tehran's nuclear programme.
Iran says it wants to resume nuclear enrichment
An Iranian official said it would resume limited uranium conversion if the 1230GMT deadline was not met.
France, Germany and the UK have said any resumption of nuclear activity would torpedo the long-running talks.
The US believes Iran is trying to build a nuclear bomb, but Iran insists its programme is for civilian use only.
Iran suspended all uranium conversion and enrichment activities in November 2004 as a result of international pressure.
However, it has always insisted that the suspension was temporary and that it would resume some of its nuclear activities regardless of EU proposals.
The EU - represented by France, Germany and the UK - has threatened to refer Iran to the UN Security Council for possible sanctions if Iran resumes its nuclear activities.
There has been no comment so far on Iran's demand for the proposals to be submitted.
Iran says the European nations had agreed to 1 August as a deadline for presenting a package of economic incentives to encourage Tehran to scale down its nuclear ambitions.
Britain, France and Germany deny they ever agreed to such a deadline, saying they merely promised to submit their proposals in late July or early August.
Iran has now brought forward the deadline to 1230GMT on Sunday.
"We will continue our talks with the EU. Iran will not resume uranium enrichment," Supreme National Security Council spokesman Ali Aghamohammadi said.
But if the deadline is missed or the proposals are not to its liking, he said Iran would resume processing uranium ore into gas at its plant in Isfahan - something it says is technically not part of the enrichment process.
By setting a deadline which it is clear Europe is not going to meet, Iran is hardening its position, says the BBC's Frances Harrison in Tehran.
But our correspondent says it is not clear if this is just a way of putting pressure on Europe before the talks or a serious threat.
Earlier this week, outgoing President Mohammad Khatami said he hoped EU diplomats would allow for a resumption of enrichment activities, but that Iran would begin again in any case.
Mahmoud Ahmadinejad, the conservative former Tehran mayor who was elected Iran's president last month, has said he wants to continue the nuclear programme.
Uranium enrichment can be used to fuel nuclear power stations, but can also provide material for nuclear weapons.