Western countries believe Iran is working to develop a nuclear bomb. Tehran says its nuclear programme is peaceful. The two days of talks in Istanbul are a follow-up to talks last month in Geneva, which were the first held in more than a year.Impatient with what some analysts have called Iran's zig-zag diplomacy, the powers are looking for a clear sign from Iran that it is ready to engage in a way that helps engender trust, even if there is no substantive progress."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."There are fundamental issues Iran needs to address and those are pretty clear, but no one's expecting any great shift.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.European Union foreign policy chief Catherine Ashton heads the delegations representing six big powers -- Britain, China, France, Germany, Russia and the United States.They were meeting Iran's nuclear negotiator Saeed Jalili.Inevitably, the two sides will go back over old ground to find a platform to move forward from."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 Tehran, if it agrees to curb its uranium enrichment activities. The Islamic state has so far showed no interest in accepting the proposals.That package, which was enhanced version of one spurned by Iran in 2006, held out prospects for civil nuclear cooperation and wider trade, for example in civil aircraft, agriculture, energy, high technology and agriculture.U.S. State Department spokesman Mark Toner said on Thursday that Washington was not expecting any "big breakthroughs", but wanted to see a constructive process emerge.The West says Iran has failed to comply with U.N. resolutions demanding it curb enrichment and grant unfettered access to its nuclear sites.The prospect of an Iranian nuclear weapon feeds fears of a Middle East conflict should the United States or Israel opt to attack it.REVISITING SWAP DEALIn their search for an opening, the powers may go back to past proposals for a nuclear fuel swap, whereby Iran would give low-enriched uranium (LEU) in exchange for higher enriched uranium (HEU) that could be used at a medical research reactor.The idea was first mooted in 2009. Since then Western powers say Iran's LEU stockpile has doubled, and it has also begun making its own HEU, so any future agreement would have to go further than was discussed in the past.Going into the Istanbul talks, Iranian officials said they were ready to discuss reviving a swap agreement based on one brokered last year with Brazil and Turkey in Tehran, which was rejected by the Western powers for not going far enough.The U.N. Security Council instead imposed a fourth round of sanctions in Iran, including steps targeting its vital oil and gas sector.The State Department's Toner said the United States was willing to discuss an updated fuel swap proposal, if it reflected the progress Tehran's has made on uranium enrichment in the two years since the idea was floated.Ahead of the Istanbul talks, the United States spoke of imposing harsher unilateral sanctions, while Russia reiterated its view the prospects for rolling back sanctions should be discussed to help persuade Iran to cooperate.