Iran and six world powers concluded their first talks in 15 months on Saturday in Istanbul, diplomats said.

World powers and Iran launched the new round of negotiations in Istanbul on Saturday, aiming to resolve a long-standing dispute over Tehran's nuclear program that threatens to spark a new war in the Middle East.

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European Union foreign policy chief Catherine Ashton on Saturday, who is heading the Western delegation at talks over Tehran's nuclear program in Istanbul, said that the talks were useful and constructive.

Ashton said that world powers are due to meet with Iranian officials for more talks on May 23, in Baghdad.

"We want now to move to a sustained process of dialogue ... We will be guided by (the) principle of step by step approach," Ashton told a news conference, calling the first meeting useful and constructive. "We will meet on May 23 in Baghdad."