It was the latest meeting in excruciatingly technical diplomatic discussions. Iran spent the day studying a new offer from the West to curb some tough international sanctions that have ravaged its economy in exchange for limiting its uranium enrichment and other activities that could be used to make weapons.
Talks in Almaty, Kazakhstan's largest city, were due to continue for a second day Wednesday, but diplomats at the negotiations warned that a major breakthrough was unlikely.
Western negotiators say they have brought small, if significant, proposals that should prove tempting to Tehran, although they have declined to divulge any details.
"The Iranians went away to consider our proposal during the course of the afternoon and the evening," said Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, who is leading the negotiations. "There was a useful discussion on that proposal."
He said Iran is "in breach of their international obligations, therefore the onus is on them to begin the process of building confidence."
Mehdi Mohammadi, a member of the Iranian delegation, said Tehran will make an offer of its own to end the deadlock but is resisting some of the West's core demands.
The Obama administration is pushing for diplomacy to solve the impasse but has not ruled out the possibility of military intervention in Iran to prevent it from acquiring a nuclear weapon.
"Our proposal includes reciprocal measures that encourage Iran to make concrete steps," U.S. Secretary of State John Kerry told reporters in Berlin. "My hope is Iran will make its choice to move down the path to a diplomatic solution."
Israel has threatened it will use all means to stop Iran from being able to build a bomb, potentially as soon as this summer, raising the specter of a possible Mideast war. In Jerusalem, former Foreign Minister Avigdor Lieberman called Tuesday on the international community to take more "significant" steps to dislodge Iran from its nuclear program. Lieberman, who is acting head of the influential Foreign Affairs and Defense Committee in Israeli's parliament, did not elaborate, but Israeli officials often hint at a possible military strike on Iran's nuclear facilities.
"Sanctions alone will not be enough to rein in those same extremists from their goal to achieve nuclear capabilities and the time has come to move toward steps that are much more significant than the talks and sanctions that we've seen to date," said Lieberman.
Lieberman resigned from his post as foreign minister in December amid legal troubles but remains a top political ally of Prime Minister Benjamin Netanyahu.
Off-and-on talks between Iran and the world powers—the United States, China, Russia, Britain, France and Germany—began after the six offered Tehran a series of incentives in 2006 exchange for a commitment from Tehran to stop enrichment and other activities that could be used to make weapons.
A senior U.S. official at the talks said some sanctions relief would be part of the offer to Iran but refused to elaborate. The official acknowledged reports earlier this month that sanctions would be eased to allow Iran's gold trade to progress, but would neither confirm nor deny they are included in the new relief offer, and spoke only on condition of anonymity to discuss the sensitive diplomatic talks more candidly.
Before the talks began Tuesday, the Interfax news agency cited Russia's envoy as saying the easing of sanctions was possible only if Iran can assure the world that its nuclear program is for exclusively peaceful purposes. China's Foreign Ministry spokeswoman, Hua Chunying, said diplomacy offered the only route to resolve the dispute and called for all sides to show flexibility, calling the Iranian nuclear issue "very complicated and sensitive."
Members of the Chinese and Iranian delegations met at a bilateral session before the main talks got under way.
Interfax cited an unidentified Iranian delegation member as saying Iran might also hold one-on-one talks with Russia, but ruled out direct negotiations with the United States in Almaty.
Officials from both sides have set low expectations for a breakthrough in Almaty—the first time the high-level negotiators have met since last June's meeting in Moscow that threatened to derail the delicate efforts.
While Mann acknowledged the Almaty talks would unlikely lead to a firm outcome, he insisted that it remained an important stepping stone toward a definitive solution.
"We're not interested in talks just for talk's sake. We're not here to talk, we're here to make concrete progress," Mann said.
The first session of talks were held in private at a hotel in Almaty and were deemed so sensitive that reporters were not allowed on the premises Tuesday save for a handful of TV cameras and photographers allowed to watch Ashton greet Saeed Jalili, secretary of Iran's Supreme National Security Council.
Tehran maintains it is enriching uranium only to make reactor fuel and medical isotopes, and insists it has a right to do so under international law. It has signaled it does not intend to stop, and U.N. nuclear inspectors last week confirmed Iran has begun a major upgrade of its program at the country's main uranium enrichment site.
Over the last eight months, the international community has imposed heavy economic sanctions on Iran, hoping they would be so painful that the Islamic republic's clerical regime would slow its nuclear program out of a moral obligation to its public. Negotiators now hope that easing some sanctions will make Tehran more agreeable to halting production of 20 percent enriched uranium—the highest grade of enrichment that Iran has acknowledged and one that experts say could be turned into warhead grade in a matter of months.
World powers also want Iran to suspend enrichment in its underground Fordo nuclear facility, and to ship its stockpile of high-grade uranium out of the country.
Mohammadi said shuttering Fordo was "out of the question" and that Iran first wants the U.N. Security Council to withdraw all of the sanctions it has heaped on the nation.
Iran has been unimpressed with earlier offers by the powers to provide it with medical isotopes and lift sanctions on spare parts for civilian airliners, and new bargaining chips that Tehran sees as minor are likely to be snubbed as well.
Iran insists, as a starting point, that world powers must recognize the republic's right to enrich uranium.
Associated Press writers Charles Hutzler in Beijing, Tia Goldenberg in Jerusalem, George Jahn in Vienna and Matt Lee in Berlin contributed to this report.