Iran opened a round of crucial nuclear talks Tuesday with proposals that it said could end an “unnecessary crisis” over its rapidly growing atomic energy program, asserting that it is prepared to take steps to alleviate Western concerns that it is seeking nuclear weapons.

The proposals, which were not immediately made public, were spelled out during a day of technical talks that Western officials described as “very useful.” It was unclear whether Iran had broken new ground in its proposals or whether the sides had narrowed differences in the decade-­long dispute over the country’s nuclear policies.

“For the first time, we had very detailed technical discussions, which carried on this afternoon,” said a senior State Department official, who spoke on the condition of anonymity to describe sensitive diplomacy.

After the formal meetings adjourned for the day, members of the U.S. delegation held rare bilateral talks with their Iranian counterparts, the official confirmed.

The one-on-one meeting followed the precedent set last month when Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif met privately in New York on the sidelines of the U.N. General Assembly. “It demonstrates our continued commitment to bilateral engagement,” the official said.

Zarif also met privately with European Union foreign policy chief Catherine Ashton late Tuesday, the second such session in as many days.

Iranian officials described Zarif’s proposals as a “road map” that laid out goals and a time­table for resolving the nuclear dispute. Using a PowerPoint presentation, the Iranian official described for more than an hour steps that he said would address Western concerns about Tehran’s nuclear activities while allowing Iran to free itself from crippling economic sanctions imposed since 2010.

Zarif gave his presentation an optimistic title: “Closing an unnecessary crisis, opening new horizons,” according to Iran’s semiofficial Fars News Agency. He insisted that details of the plan be kept strictly confidential while the discussions were underway.

Western officials described the tone of the talks as positive but offered few details.

“We heard a presentation this morning from Foreign Minister Zarif. It was very useful,” said Michael Mann, a spokesman for Ashton. Formal negotiations were expected to resume Wednesday.

A spokesman for the Iranian team, Deputy Foreign Minister Seyed Abbas Araghchi, said afterward that “serious” talks were conducted in a “highly positive atmosphere.” He suggested that the Geneva negotiations would be quickly followed by more meetings, in rapid succession, with the goal of reaching a deal on an accelerated timetable.

“Iran does not want to move in an ambiguous atmosphere,” he told reporters.

The statements added to an air of expectancy surrounding the talks, the first negotiations on Iran’s nuclear program since the June election that propelled moderate cleric Hassan Rouhani to the presidency. Rouhani, who campaigned on a pledge to end Iran’s international isolation and gain relief from the sanctions, has embarked on a diplomatic charm offensive in recent weeks, capped by last month’s 15-minute phone call with President Obama, the first direct contact between U.S. and Iranian leaders in three decades.

The United States is negotiating with Iran as part of a six-
nation bloc that includes Britain, China, France, Germany and Russia. The group failed to secure a deal with Iran in multiple rounds of negotiations over the past two years, as Iran continued to rapidly expand its capacity to produce enriched uranium. Western governments fear that Iran will soon possess enough fissile material to become a virtual nuclear power, needing only a few days or weeks to assemble a nuclear bomb if it decides to make one.

In the past, Iranian officials dismissed such concerns, insisting that the country’s nuclear program is peaceful. But in another shift, senior government officials in recent days have acknowledged that Western concerns should be addressed.

“Despite the fact that we don’t accept the basics of the Westerners’ concerns and don’t assume them to be fair, we can obviate them using international mechanisms, treaties, and laws and regulations,” Ali Akbar Salehi, the head of Iran’s atomic energy agency, said Tuesday, according to Fars.

Although details about Iran’s Geneva proposals were sparse, U.S. arms-control experts welcomed the prospect of a diplomatic thaw.

“It would appear that the two sides are within striking distance of a framework agreement before the end of the year,” said Daryl Kimball, director of the Arms Control Association, a Washington-based nonprofit group.

But many congressional officials remained skeptical. A bipartisan group of 10 U.S. senators warned Obama in a letter ahead of the talks that they would oppose any arrangement that allowed Iran to retain the right to enrich uranium on its own soil.

“If Iranian actions fail to match the rhetorical reassurances of the last two weeks, we are prepared to move forward with new sanctions to increase pressure on the government in Tehran,” warned the letter, whose signatories included such prominent Democrats as Sen. Robert Menendez (N.J.), chairman of the Senate Foreign Relations Committee.

That threat brought an equally strong response from conservatives in Iran’s parliament. Jalil Jafari, who heads the parliament’s energy committee, told Fars that Iranian lawmakers would not approve a deal that did not recognize Iran’s right to enrich uranium for peaceful purposes.

“The Iranian negotiators know that the rights of the Iranian nation — maintaining uranium enrichment and nuclear technology — are not something to be ignored,” he said.