Nuclear talks between Iran and six world powers began on a discordant note in Kazakhstan on Friday as Western officials criticized the Islamic republic for failing to respond substantively to demands that it scale back its nuclear ambitions.

Negotiations opened in the Kazakh city of Almaty with Iranian diplomats unveiling what they described as a “comprehensive proposal” for ending international tensions over Iran’s advances in nuclear technology. The deputy leader of the Iranian delegation, Ali Baqeri, said at a news conference that the new offer was intended to allay concerns and “establish a new bedrock for cooperation.”

But Western diplomats, after a break in the talks, said the Iranians put forward only a vague proposal that reflected little change from previous positions. A few additional details were extracted during an afternoon session before the sides adjourned for the night, said a Western diplomat who spoke on the condition of anonymity to describe the give-and-take inside the meeting room at a posh Almaty hotel.

“We had a long and substantial discussion on the issues, but we remain a long way apart on the substance,” the official said. “We are now evaluating the situation and will meet again tomorrow.”

In Washington, State Department spokeswoman Victoria Nuland suggested that the afternoon meeting had been more productive, although no agreement had been reached.

“They obviously considered that it was worth coming back tomorrow to see what more could be done. Otherwise, they wouldn’t have done that,” she said.

U.S. and European diplomats had expected a detailed response from Iran to a set of proposals designed to lower tensions in the increasingly volatile standoff over Iran’s nuclear program. Those proposals, which included a number of concessions favorable to the Islamic republic, were hailed by a senior Iranian diplomat as a possible “turning point” during the last round of nuclear talks in late February.

But hopes for further progress appeared to fade during the opening hours of Friday’s talks. Instead of a detailed counter­offer, the Iranians appeared to be repeating talking points from previous rounds of negotiations, the Western diplomat said.

“There has not yet been a clear and concrete response,” said the diplomat, who described his colleagues as “somewhat puzzled” by what the Iranians had put on the table. “There were some interesting but not fully explained general comments on our ideas.”

The six world powers at the talks — the United States, Britain, China, France, Germany and Russia — have called on Iran to agree to limits on its nuclear program, including a freeze on production of a type of enriched uranium that is considered close to weapons-grade. Such a freeze would be regarded as a ­“confidence-building measure” that would set the stage for a comprehensive agreement with multiple safeguards to ensure that Iran does not use its nuclear infrastructure to build atomic weapons.

In exchange for the freeze, Iran was offered relief from some economic sanctions, with the promise of a more substantial easing in the future.

Iran, which insists that its nuclear program is exclusively for peaceful purposes, has demanded immediate relief from sanctions as part of any deal. Its proposal Friday sought to link any confidence-building steps to a broader agreement that would include international recognition of Iran’s right to enrich uranium, according to diplomats familiar with the offer.