Talks on Iran’s nuclear program sputtered to a halt Saturday after diplomats reported scant progress in narrowing differences over proposals to limit Iran’s atomic advances in exchange for sanctions relief.

The negotiations in Almaty, Kazakhstan, ended in a frustrating muddle, with the sides describing the talks as constructive but ultimately fruitless in resolving a dispute that has raised fears of another military conflict in the Middle East. At the conclusion, the negotiators could not agree on dates for a new round of meetings, nor could they declare diplomacy at an impasse.

“Over two days of talks we had long and intensive discussions on the issues,” Catherine Ashton, the European Union’s foreign policy chief, said at a news conference as the talks adjourned. “It became clear that our positions remained far apart.”

Yet, despite the gloomy assessment, Western diplomats said the talks were useful, allowing a robust if sometimes contentious airing of differences between Iran and the six world powers that are seeking to extract nuclear concessions from the Islamic Republic.

A Russian diplomat at the talks, Deputy Foreign Minister Sergei Ryabkov, called the negotiations “certainly . . . a step forward,” and a senior U.S. official described “quite substantive” exchanges with Iran as the discussions entered their second day.

“It is fair to say that both sides came away with a better understanding of the other’s thinking,” said the U.S. official, speaking on the condition of anonymity to describe sensitive diplomacy.

Acknowledging a “significant gulf” between the sides, the official said decisions on next moves would not be clear until the diplomats return to their capitals for consultations. “All of us need to evaluate what the next steps should be in this process and think through how we can more effectively get there,” the official said.

Iran’s chief negotiator, Saeed Jalili, alluded to remaining obstacles to an agreement in a brief news conference in which he repeatedly insisted on international recognition of Iran’s right to make enriched uranium. Iranian officials have suggested they might accept demands that the country freeze production of some of its enriched uranium as a “confidence­-building” measure, but only if the West immediately lifts economic sanctions that have battered the country’s economy.

“Confidence-building is a two-way road and should include reciprocal and balanced actions,” Jalili told reporters. “It should not include depriving a nation of its rights, rather it should be based on mutual cooperation.”

The talks appeared to be lurching toward failure within hours after the start of the meetings in a posh Almaty hotel. The six nations in the talks with Iran — the United States, Britain, China, France, Germany and Russia — had called on Iran to respond to a detailed proposal that included halting production of a type of enriched uranium that is considered close to weapons-grade. In exchange, Iran was offered relief from some economic sanctions, with the promise of a more substantial easing in the future under a comprehensive deal aimed at lessening international anxiety over Iran’s nuclear intentions.

On Friday, Iranian diplomats unveiled what they described as a “comprehensive proposal” for settling the dispute, one that hinged largely on the demand for sanctions relief and recognition of Iran’s nuclear rights. But Western officials expressed bafflement with the offer, which diplomats said was merely a reworked version of Iran’s previous positions.

The failure of the sides to achieve tangible progress — or even a firm date for new talks — was seen as a disappointment for the Obama administration, though most experts had anticipated at best modest progress in nuclear talks until after Iran’s presidential election in June. Some experts said the results in Almaty will increase pressure on the White House to further tighten the economic noose on Tehran.

“With another failed round of negotiations, Iran’s supreme leader, Ali Khamenei, is proving once again that he is not interested in compromise,” said Mark Dubowitz, executive director of the Foundation for Defense of Democracies, a Washington policy institute. “He continues to assess that Iranian nuclear physics is beating Western economic pressure.”

Trita Parsi, an Iran expert who is critical of U.S. sanctions policy, faulted both sides for continuing to talk past each other.

“While the U.S. has been focused on red lines, in Kazakhstan, the Iranians were looking for the finishing line,” said Parsi, author of “A Single Roll of the Dice,” a book on the Obama administration's Iran policies. “Ultimately, all sides must take risks for peace.”