Senior officials from France, Britain and Germany have offered Iran a last chance to abandon uranium enrichment in return for other concessions, or face UN sanctions.
The EU's ‘Big Three’ had reluctant support from the US for the offer, despite Washington’s belief that Iran is using the talks to buy time to acquire the capability to build a nuclear bomb.
The head of Iran's delegation, Amir Zamaninia, declined to comment to reporters as he arrived for the meeting. The Iranian delegation was due to present a counter-proposal, though details were unclear.
Iran says its nuclear programme is only for power generation and that it will never give up uranium enrichment.
If Iran rejects the EU offer, most European nations are likely to back US demands that Tehran be reported to the UN Security Council.
Some diplomats say Iranian officials have never clearly explained why their oil-rich state needs nuclear energy or why they are so intent on producing nuclear fuel.
The Europeans are offering to support the building of light-water reactor systems if Iran will scrap plans to build a heavy-water research reactor.
Other incentives in the European offer include resumption of talks on an EU-Iran trade pact and guarantees of Russian fuel.