Six world powers will attempt to persuade Iran to rein in its nuclear programme as talks take place in Turkey, but there is little expectation of tangible results beyond an agreement on a framework for further negotiations.
"We're not expecting any big breakthroughs but we want to see a constructive process emerge that ... leads to Iran engaging with the international community in a credible process and addressing the international community's concerns about its nuclear programme," Mark Toner, a US state department spokesman, said ahead of Friday's talks.
The five permanent members of the UN Security Council - Britain, China, France, Russia and the United States - plus Germany have said that they believe Iran's uranium enrichment process is a cover for developing nuclear weapons.
Iran has repeatedly denied the allegations, saying it is developing the the technology to meet its civilian energy needs.
The standoff over the country's nuclear programme has escalated in the past year, with the United Nations imposing new sanctions and the US and its allies rejecting a revised proposal for Iran to exchange low-enrichment uranium for nuclear fuel from abroad.
Sanctions issue
Sergei Lavrov, Russia's representative at the talks, said that the lifting of the "counterproductive" sanctions should be on the table on Friday.
"The nuclear programme must be at the heart of the discussions and the problems that have not yet been resolved concerning it," Lavrov said at a joint press conference with his Turkish counterpart.
"But there's not only one topic for this meeting, the lifting of sanctions on Iran must also be on the agenda."
However, Toner dismissed Lavrov's remarks.
"I think that UN Security Council Resolution 1929 stipulates what Iran needs to comply with in order to have those sanctions lifted," he said.
The US has raised the possibility of harsher unilateral sanctions if Iran fails to comply with the demands of the six powers.
Going into the Istanbul talks, Iranian officials said they were ready to discuss reviving the idea of the swap agreement based on one brokered last year with Brazil and Turkey in Tehran.
Toner said the United States was willing to discuss an updated fuel swap proposal, if it reflected the progress Tehran has made on uranium enrichment in the two years since the idea was floated.
Iran defiant
On the eve of the talks, Mahmoud Ahmadinejad, the Iranian president, was defiant, telling a cheering crowd that Tehran would not back down on the issue of its nuclear programme.
"They say: 'We want negotiation' ... You are free to choose the path (of either cooperation or confrontation), but bear in mind that by adopting the old path (of confrontation), you will face a more scandalous defeat," he said.
"You could not stop us from being nuclear ... The Iranian nation will not retreat an inch. The nuclear issue is over from the Iranian point of view."
Ali Asghar Soltanieh, Iran's envoy to the International Atomic Energy Agency (IAEA), also delivered an aggresive message, saying Iran's enrichment activities would continue at an underground facility at Fordow, near the city of Qom, even if its nuclear sites were attacked.
