EU foreign policy chief Catherine Ashton said negotiators will meet in Baghdad next month. Chief Iran negotiator Saeed Jalili said next round should focus on confidence-building measures.
Iran and world powers discussed Tehran's controversial nuclear program for the first time in more than a year on Saturday and, in what Western diplomats called a constructive development given their low expectations, agreed to meet again next month.
Catherine Ashton, the European Union foreign policy chief who has headed negotiations for the six world powers present, told a news conference after a day of talks in Istanbul that they had arranged to meet the Iranian delegation again in Baghdad on May 23.
Saeed Jalili, the chief Iranian negotiator, told a news conference there had been differences of opinion but that some important points had been agreed to. He said that the next talks should focus on arranging measures to build mutual confidence.
Western participants had said previously that agreeing to meet for a second round of talks would constitute a successful day. It may remove some heat from a crisis in which warnings from Israel of a possible strike against Iranian facilities have stoked fears of a major war in an already unsettled Middle East.
After a day in which diplomats had spoken of a more engaged tone from Iranian officials, compared to the 15 months of angry rhetoric since the last meetings, Ashton characterized the talks positively.
"The discussion on the Iranian nuclear issue have been constructive and useful," she told a news conference. "We want now to move to a sustained process of serious dialogue, where we can take urgent, practical steps to build confidence."
One non-Iranian diplomat called the atmosphere "completely different" from that of previous meetings, as Western delegates watched for signs that Iran was ready to engage after more than a year of threats in defense of its right to pursue nuclear energy and denials it wants to be able to build an atom bomb.
Ashton said that the negotiating powers wanted Iran to meet international obligations - it is a signatory to the treaty that prevents the spread of nuclear weapons - and should reciprocate in negotiations.
The West accuses Iran of trying to develop a nuclear weapons capability. Israel - believed to be the only Middle East state with an atomic arsenal - sees Iran 's atomic plans as a threat to its existence and has threatened military action.
Iran says its program is peaceful and has threatened to retaliate for any attack by closing a major oil shipping route.
The United States and Israel have not ruled out military action to destroy Iran's nuclear sites.
The morning round of talks was "completely different" from the previous meeting 15 months ago and Iranian chief nuclear negotiator Saeed Jalili had not stated the kind of preconditions that he did in the last meeting in early 2011, a diplomat said.
"He seems to have come with an objective to get into a process which is a serious process," the envoy said. "I would say it has been a useful morning's work."
Iran had said it would propose "new initiatives" in Istanbul, though it was unclear whether it is now prepared to discuss curbs to its enrichment program.
"They met in a constructive atmosphere," said Michael Mann, a spokesman for EU foreign policy chief Catherine Ashton, after the morning session of talks. "We had a positive feeling that they did want to engage."
Iran, one of the world's largest oil producers, says its nuclear program has solely peaceful objectives - to generate electricity and produce medical isotopes for cancer patients.
But its refusal to halt nuclear work that can have both civilian and military uses has been punished with intensifying US and EU sanctions against its oil exports.
"Given that oil revenue accounts for over half of government income, the budget will be under significant strain this year as oil exports fall as a result of sanctions and oil production is cut back by Iran as its pool of buyers begins to shrink," says Dubai-based independent analyst Mohammed Shakeel.
Western officials have made clear their immediate priority is to persuade Tehran to cease the higher-grade uranium enrichment it began in 2010. It has since expanded that work, shortening the time it would need for any weapons "break-out."
Iran has signaled some flexibility over limiting its uranium enrichment to a fissile purity of 20 percent - compared with the 5 percent level required for nuclear power plants - but also suggests it is not ready to do so yet.