The first talks between Iran and six world powers for more than a year have been described as positive and a new round has been agreed to in May.
Envoys from the United States, Britain, France, China, Russia and Germany met Iranian representatives in Istanbul on Saturday, breaking a 15-month impasse.
European Union foreign policy chief Catherine Ashton described the talks as "constructive and useful".
But some diplomats said there was still a long way to go, and specific actions were needed from Iran.
Baroness Ashton said Iran has the right to a peaceful nuclear programme and that the nuclear Non-Proliferation Treaty must be a "key basis" for future talks.
Iran's chief negotiator, Saeed Jalili, said the talks had been being based on co-operation and were "very successful".
He said next month's meetings should focus on building mutual confidence.
Iran believes that it is entitled to enrich uranium up to 20% under the Non-Proliferation Treaty.
The enrichment is to a higher quality than needed to produce electricity.
Iran says the programme is for medical and other peaceful uses but critics suspect it of trying to develop nuclear weapons.
Israel has hinted in recent months that it may carry out a pre-emptive strike, the BBC reports.
A White House spokesperson praised the "positive attitude" from Iran, describing Saturday's talks as a first step.
The next round of talks is scheduled for 23 May.