Participants are expected to revive a fuel-swap plan in which Iran surrenders low-enriched uranium for fuel for a research reactor, the BBC reported.
European Union foreign policy director Catherine Ashton heads the delegation from Britain, China, France, Germany, Russia and the United States tasked with trying to get Iran to be more transparent about its nuclear ambitions.
Iran, however, has insisted that uranium enrichment won't be discussed in Istanbul, pushing for talks on global disarmament and Israel's suspected nuclear arsenal.
"We want to discuss the fundamental problems of global politics at the Istanbul talks," said Saeed Jalili, head of the Iranian delegation.
U.S. State Department spokesman Mark Toner said Western officials aren't "expecting any big breakthroughs."
Since expectations are low, Western diplomats said their primary goal is to try to persuade Iran to get rid of its stockpile of low-enriched uranium, estimated to be about three tons, which could be enough for several nuclear weapons if further enriched.
Under the arrangement, Iran would give up an agreed-to amount of its low-enriched uranium. In return the world powers would provide fuel for a research reactor in Tehran. Two previous attempts to strike a uranium-for-nuclear fuel deal failed.
Meanwhile, The Guardian published a leaked U.S. diplomatic cable from 2009 indicating U.S. experts at a conference in Vienna said they believed Iran had the "technical ability" to make highly enriched uranium for use in nuclear weapons. The cable also indicated other international experts disagreed.
During the 2009 meeting, a U.S. representative said, "Iran had now demonstrated centrifuge operations such that it had the technical ability to produce highly enriched uranium if it so chose," the cable read.