EU foreign policy chief Catherine Ashton says the positions of six world powers and Iran "remain far apart" on Tehran's nuclear program despite two days of long and intensive discussions in Almaty, Kazakhstan.
Nonetheless, she appeared to indicate that the talks had been constructive.
"For the first time that I've seen, [there has been] a real back and forward between us, where we're able to discuss details, to pose questions, and to get answers directly on the sort of issues that we need to discuss in order to be able to move forward," she said. "To that extent, that has been a very important element. However, what matters in the end is substance and in that we are still a considerable way apart."
Ashton added that all sides had agreed to "go back to their capitals to evaluate where we stand in the process."
Russian Deputy Foreign Minister Sergei Ryabkov said the two sides had failed to find "mutual understanding" and that a time and place for the next talks remained to be agreed.
Their comments came after the two sides had met for a full day on April 6 over the Iranian nuclear crisis, beginning with bilateral meetings between Ashton and chief Iranian negotiator Said Jalili.
Iranian diplomats also met separately with the Russian and Chinese delegations.
The six world powers -- the United States, Russia, China, France, Britain, and Germany, the so-called P5+1
-- were meeting with Iran for the second time in five weeks in Almaty.
At the first meeting in February, the P5+1 offered to lighten some economic sanctions on Iran if Tehran agreed to suspend higher-grade uranium production.
On the first day of this round of talks on April 5, Iran's deputy negotiator Ali Bagheri did not indicate whether the offer of the six states was acceptable but said the Iranian side had given a "detailed response to all the questions."
Nonetheless, some unnamed Western diplomats told Western media that they were "puzzled" by Iran's failure to respond to the Western offer.
However, Russia's Sergei Ryabkov, said Tehran was showing "some seriousness" in the negotiations.
The talks have been held amid warnings from Israel that it could bomb Iranian nuclear installations if diplomacy and sanctions fail to curb Tehran's nuclear progress.
Ahead of the Almaty talks, Jalili said the chances of a breakthrough hinged on the world powers recognizing Iran's right to enrich uranium.
The UN Security Council, however, has demanded Iran stop the process in several resolutions dating back to 2006.
Iran has denied it is secretly developing nuclear weapons, as the West claims, and says its nuclear program is solely for energy generation and medical research.
With reporting by AFP, Reuters, and AP