MOSCOW — Iran on Monday offered up a blistering critique of a proposal by six world powers to rein in its nuclear program, marking the latest setback in efforts to find a diplomatic solution to the conflict.

In Iran’s first detailed analysis of the proposal, the nation’s chief negotiator, Saeed Jalili, ticked off a list of objections in a five-hour negotiating session at a Moscow hotel and expounded at length about Tehran’s grievances with the West, dating back to at least 1968.

The meeting, the third this year between Tehran and the six powers, “was intense, it was tough,” said Michael Mann, a spokesman for the European Union’s foreign policy chief, Catherine Ashton. He said the Iranians were finally addressing the proposal directly, but “it was not discussed in quite the way we had hoped at this stage.”

He said both sides would “reflect” overnight and meet Tuesday to consider their next step and whether it still made sense to schedule another round of talks.

Diplomats for the six powers — Britain, France, Russia, China, Germany and the United States —said the Iranians’ objections may not be as much of a blow to the talks as they appear, noting that Iran sometimes turns conciliatory after behaving belligerently.

“They have reason to want these talks to continue too,” said a senior Western diplomat who requested anonymity, which is common during such negotiations.

Nevertheless, the Iranian critique, rolled out in a PowerPoint presentation, extended a losing streak for the negotiations. Last month in Baghdad, the Iranians dashed hopes for early progress in the talks by complaining bitterly in a session about their treatment on the nuclear issue and cataloging grievances unrelated to the topic.

Even a temporary halt in the talks could upset nervous oil markets and renew talk of an air attack by Israel to end the threat it sees from the Iranian nuclear program.

Iran’s resistance may stiffen U.S. congressional objections to the talks. Forty-four senators, including some liberal Democrats, sent a letter to President Obama on Friday saying he should reconsider the talks if there was no tangible progress in Moscow.

Diplomats for the six countries declined to detail what Iran didn’t like about the proposal. The plan seeks to begin a lengthy negotiation with a temporary solution by calling for Iran to halt production of uranium enriched to 20% purity, dismantle an underground site where the material is being produced and send the existing stockpile out of the country.

Uranium enriched to 20% is a special source of worry because it can be brought with relative ease to the 90% purity needed to make fuel for a nuclear bomb.

During the session, Iran pressed for economic sanctions on the nation to be lifted and for recognition of its right to enrich uranium on its soil. Western sanctions on Iran, already tough, become more stringent at the end of the month with a new round of measures, including a European Union ban on all imports of Iranian oil.

Iranian officials have made a string of critical comments about the process to the news media in recent days. One told reporters that so far the atmosphere for the meeting was “not positive at all.”

Despite the complaints, Ali Bagheri, the deputy Iranian negotiator, said late in the day Monday that the talks had been “serious and constructive.”

Jalili laid out in his presentation what the Iranians call their five-point plan. It consists of five sets of issues surrounding Iran’s right to enrich uranium, its fractious relations with the United Nations’ nuclear watchdog agency, the interim plan proposed by the six powers, proposals for civil nuclear cooperation, and nonnuclear issues such as how to handle the uprising in Syria.

The day’s session began with the six powers laying out their proposal, which includes a number of incentives for Iranian cooperation. The group has offered to provide help with nuclear safety and civil nuclear power, as well as aircraft parts Iran needs.

The group is to meet again at noon Tuesday, with an opportunity for the Iranians to hold one-on-one meetings with other delegations beforehand. The Iranians have so far declined opportunities to meet with the U.S. diplomatic team.

Jalili left the hotel shortly after 6 p.m. to have dinner with senior Russian officials, including Nikolai Patrushev, secretary of the national security council and former head of the FSB intelligence agency. Russia has special influence with Iran, and some diplomats held out hope that the dinner could turn the discussions in a more favorable direction.

Though the Western powers are at odds with Iran and Russia over the strife in Syria, diplomats said the dispute was not a complication in the talks.

paul.richter@latimes.com