Iran has agreed to suspend most of its uranium enrichment, as part of a deal with the EU aimed at resolving the dispute over its nuclear programme.
Iran says its nuclear programme is designed for energy use only
Chief Iranian negotiator Hassan Rohani said the suspension would be in force until a final settlement is reached.
The EU has offered Iran increased co-operation on trade and energy in exchange for the freeze.
Iran insists its nuclear activities are peaceful, but the US says they are part of a secret weapons programme.
Successful uranium enrichment could be seen as a key stage in the development of weapons-grade nuclear material.
Speaking in Tehran, Mr Rohani said Iran would suspend "almost all" its enrichment activities until a long-term agreement on Iran's nuclear programme is reached.
Talks will begin next month, he added.
The International Atomic Energy Agency (IAEA) - the UN's Vienna-based nuclear watchdog - passed a resolution in September calling on Iran to stop enriching uranium.
Once confirmed by IAEA inspectors on the ground, the freeze means the agency's board it unlikely to take the dispute to the UN Security Council for possible sanctions when it meets later this month.
A deal reached last year between the EU and Iran on a uranium-enrichment freeze later unravelled.
Iran has said it has a legal right to nuclear energy - and in particular to securing their own source of fuel for power stations, rather than being dependent on outsiders.