Catherine Ashton said Saturday that the talks revealed that "the two sides remain far apart on substance."
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Nevertheless, a senior US official said there had been no breakdown in negotiations, despite the lack of substantive progress.
"There was no breakthrough but also no breakdown," the official, who declined to be identified, said after the meeting in the Kazakh city of Almaty. The official said the major powers intended to proceed with diplomatic efforts to solve the dispute.
"Over two days of talks we had long and intensive discussions on the issues addressed in our confidence-building proposal," Ashton told a news conference in the Kazakh city of Almaty, where the negotiations took place.
Russia's negotiator sounded more upbeat, saying the talks were "definitely a step forward" although no compromise had been reached, Interfax news agency reported, without giving details.
Representatives of Iran and six world powers around negotiating table (Photo: EPA)
Iran's chief nuclear negotiator said there was some distance between the positions of Tehran and world powers but its disputed uranium enrichment could be a subject for confidence-building cooperation.
"We proposed our plan of action and the other party was not ready and they asked for some time to study the idea," Jalili told a news conference. He said was now up to the powers to demonstrate willingness to take confidence-building steps.
Ashton said that for the first time there had been a "real back and forth between us when were able to discuss details, to pose questions, and to get answers directly ... To that extent, that has been a very important element"
But, she added: "What matters in the end is substance. We know what we want to achieve and the challenge is to get real engagement so we can move forward with this and that's the ambition."
The six insist Iran cut back on its highest grade uranium enrichment production and stockpile, fearing Tehran will divert it from making nuclear fuel to form the material used in the core of nuclear warhead.
Iran insists it has a right to enrich but says it has no interest to use the technology to make weapons. It wants more sanctions relief than the six are offering for any concessions on its part.
Reuters and AP contributed to this report
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