Iran gave no sign of making concessions to world powers bent on coaxing it to curb its nuclear programme at talks on Friday, saying it would not discuss suspending sensitive uranium enrichment.
Western leaders suspect Iran is working covertly to develop a nuclear weapon. Tehran says its atomic energy programme is peaceful. The two days of talks in Istanbul are a follow-up to talks last month in Geneva, the first held in more than a year.
Impatient with what some analysts have called Iran's zigzag diplomacy, the powers are looking for a clear sign from Tehran that it is ready to engage in a way that helps engender trust, even if there is no substantive progress.
Iran's National Security Council issued a statement, quoted by Iranian television, saying the first session of talks on Friday was held in a positive atmosphere. Iran's nuclear negotiator Saeed Jalili is the council's secretary general.
One of Jalili's aides in Istanbul drew a red line round its enrichment activities during the meeting. Uranium enriched to a low degree yields fuel for electricity or, if refined to a very high level, the fissile core of a nuclear bomb.
We will not allow any talks linked to freezing or suspending Iran's enrichment activities to be discussed at the meeting in Istanbul, Abolfazl Zohrevand said.
So far this issue has not been discussed, has not been raised or mentioned by the other party, Zohrevand said.
Iran's nuclear rights cannot be discussed.
European Union foreign policy chief Catherine Ashton heads the delegations representing six big powers -- Britain, China, France, Germany, Russia and the United States.
No one is expecting any big breakthrough, but Iran needs to show that it is interested in engaging in a wider process, said one diplomat as the opening session began.
Iran's nuclear standoff with the West has escalated in the past year, with the United Nations imposing new sanctions and Western states rejecting a revised proposal for Iran to swap some of its fuel abroad as too little, too late.
It is very important that Iran takes those negotiations seriously, that it is prepared to discuss its nuclear programme in detail, Foreign Minister William Hague said during a news conference in New Zealand.
These negotiations are a test of Iran's willingness to enter into, and to keep to its international obligations under the Non-Proliferation Treaty, and under successive resolutions of the U.N. Security Council, Hague said.
Iran has ignored Security Council resolutions demanding it suspend enrichment, with trade and other benefits offered in return, and grant unfettered access for U.N. nuclear inspectors.
The prospect of an Iranian atom bomb fans fears of a broader Middle East conflict should the United States or Israel opt to attack it, a mooted last-ditch option should diplomacy fail.
BACK TO OLD OFFERS
The first session of two hours on Friday morning ended with the two sides looking for common ground.
An EU official said Ashton was to meet Jalili shortly to discuss how to advance the talks, with the aim of persuading him to agree to a series of bilateral meetings, especially with the United States, with whom Iran has had no relations for 30 years.
The existing offer from 2008 to build trust is still on the table. Essentially we will reiterate that, the Western diplomat said. He was referring to a package of economic, political and other incentives offered to the Islamic Republic if it agrees to mothball enrichment-related activities.
That package, an enhanced version of one spurned by Iran in 2006, held out prospects for civil nuclear cooperation and trade in civil aircraft, agriculture, energy and high technology.
In their search for an opening, the powers may go back to proposals for a nuclear fuel swap, whereby Iran would part with low-enriched uranium (LEU) in exchange for highly processed fuel to keep a Tehran reactor that makes medical isotopes running.
The goal, for the six powers, was to divest Iran of enough LEU to delay it accumulating enough for a nuclear weapon while negotiations proceeded on a broader solution to the crisis.
The idea was first tentatively agreed in October 2009 only for Iran to back out some weeks later. Since then, Iran's known LEU stockpile has doubled and it has begun enriching uranium up to 20 percent fissile purity for conversion into reactor fuel. Weapons-grade uranium requires 90 percent enrichment.
So for the powers, any future swap would have to go further, involving much more than the 1,200 kg of LEU agreed in 2009. Iran has indicated readiness to revive only the original deal.
A Western diplomat said there was an impasse as Iran was setting conditions, most notably a suspension of sanctions.