The first discussions between Iran and six world powers on Tehran's nuclear programme have ended on a positive note, Western officials have said.
But some diplomats said there was still a long way to go, and specific actions were needed from Iran.
The talks in Istanbul on Saturday were the first for 15 months. The next round will take place in Baghdad on 23 May.
Iran says its programme is peaceful, but critics suspect it of seeking to develop a nuclear weapons programme.
The European Union's foreign policy chief, Catherine Ashton, described the talks as "constructive and useful".
Speaking at the end of the talks between the six countries - the US, UK, France, China, Russia and Germany - and Iran, Baroness Ashton said that future discussions would be guided by the "principle of a step-by-step approach and reciprocity".
She said that Iran has the right to a peaceful nuclear programme, and that the nuclear Non-Proliferation Treaty must be a "key basis" for future talks.
The Iranian chief nuclear negotiator, Saeed Jalili, described Saturday's talks as being based on co-operation and "very successful".
He said next month's talks should focus on building mutual confidence.
Iran believes that it is entitled to enrich uranium up to 20% under the Non-Proliferation Treaty. Mr Jalili said that Tehran expects "that we should enjoy our rights in parallel with our obligations".
The British Foreign Secretary, William Hague, welcomed the talks, but said there was "a long way to go" to resolve the dispute.
The French Foreign Minister, Alain Juppe, said Iran needed "to make urgent and concrete gestures to establish confidence" in the next round of talks.
A White House spokesperson praised the "positive attitude" from Iran, describing Saturday's talks as a "first step".
The world powers meeting Iran in Istanbul hope eventually to persuade the country to reduce its enrichment of uranium and fully open up its nuclear facilities to inspectors from the International Atomic Energy Agency (IAEA).
There are suggestions that the stringent sanctions on Iran could be reduced if it complies with the requests.
Israel - which sees a potential Iranian nuclear arsenal as a threat to its existence - has hinted in recent months that it may carry out a pre-emptive strike.
US President Barack Obama earlier described the talks as a "last chance" for diplomacy to work.
The BBC's Iran correspondent, James Reynolds, following the talks in Istanbul, says the central issue which underlies the nuclear argument between Iran and the West is the lack of trust between the two sides.
Overcoming the legacy of decades of suspicion and mistrust may take more than just a handful of meetings, he says.