Six nations have started talks with Iran in an attempt to satisfy both Tehran's demands for international recognition of its right to advanced nuclear technology and world concerns that the Islamic Republic wants to misuse that expertise to make atomic arms.
The two sides parted in February after a previous meeting in the Kazakh city of Almaty with agreement to at least keep talking over a new proposal submitted by the six - the United States, the UK, Russia, China, France and Germany. But they remain vastly divided on what they want from each other.
Iran wants an end to punishing sanctions crippling its economy. They were imposed to force it to end uranium enrichment, a process that can generate nuclear energy and the core of nuclear weapons. Iran denies any interest in atomic arms, insists its enrichment programme serves only peaceful needs, says it has a right to enrich under the Nuclear Non-proliferation Treaty and describes UN Security Council demands that it stop enrichment as illegal.
"We are talking about peaceful nuclear energy," Saeed Jalili, Iran's chief nuclear negotiator, said before the two sides sat down at one of Almaty's five-star hotels. He said Iran had a right to such a programme and accused "a handful of countries" of working "to deny this right to others".
The six have moved from demanding a total end to enrichment, and are asking Tehran only to stop production and stockpiling of uranium enriched to 20%, which is a technical step away from weapons-grade uranium. A halt to production and stockpiling would keep Iran's supply below the amount needed for further processing into a weapon.
Ahead of the meeting an EU official speaking for the six world powers said the onus was on Iran to engage on the six-nation offer, which foresees a lifting of some sanctions but keeps penalties crippling Iran's oil sales and economy in place.
"What we are hoping for is that Tehran will come back to us today with a clear and concrete response," said Michael Mann.
"The core issue here is the international community concern of the very strong indications that Iran is developing technology that could be used for military purposes," said Mr Mann, spokesman for EU foreign policy chief Catherine Ashton, the formal convener of the talks.
"There are suspicions of an enrichment programme that could have military uses," he said. "The confidence building has to come from Iran because it is Iran that is developing its nuclear programme."
Ahead of today's session, a senior US administration official suggested more punitive sanctions would be imposed if "Iran does not begin to take concrete steps and concrete actions to meet international concerns".
The meeting broke for lunch after about three hours with no indication whether progress had been made.
The meetings today and tomorrow are at best expected to achieve enough progress for agreement to hold another round of talks, but after 10 years of inconclusive negotiations, even an agreement to keep talking would give both sides short-term gains.
It would leave the international community with some breathing space in its efforts to stem Iran's nuclear advance. For Tehran, continued negotiations are insurance that neither Israel nor the United States will feel the need to act on threats to move from diplomacy to other means.
Israel says Iran is only a few months away from the threshold of having material to turn into a bomb and has vowed to use all means to prevent it from reaching that point. The United States has not said what its "red line" is, but has said it will not tolerate an Iran armed with nuclear weapons.
Any strike on Iran could provoke fierce retaliation directly from Iran and through its Middle East proxies in Syria, Lebanon and Palestine, raising the spectre of a larger Middle East conflict.