The six powers China, Britain, France, Russia, the United States, and Germany want to nudge Iran toward acknowledging the need to reduce worries that it might shift its uranium enrichment program into making weapons.
"Compared to the Geneva talks, the negotiations in Istanbul are being held in a more positive way," Iranian delegate Abolfazl Zohrevand said, referring to talks in the Swiss city that ended last month with only an agreement to meet again in Turkey. "There are good signs that the two sides will make progress."
He told The Associated Press that compromise by Iran's negotiating partners was moving the talks forward.
"They didn't get what they had hoped to get from pressure and sanctions," he said. "They are showing some flexibility. This is helping both sides to be optimistic."
But U.S. officials in Washington were unimpressed with Iran's presentation and held out little hope for progress in convincing Iran to comply with international demands in two days of talks.
A major accomplishment, the officials said, would be an agreement for another round of talks. They spoke on condition of anonymity due to the delicacy of the diplomacy and because the meeting was not over.
A diplomat familiar with the talks said the two sides stated their positions then broke for lunch a buffet of chicken saltimbocca with smoked tomato squash, steamed seabass, ravioli, baldo pilaf, grilled vegetables and deserts. The diplomat asked for anonymity in exchange for commenting on the closed meeting.
Tehran denies that it wants nuclear weapons, insisting it wants only to make peaceful nuclear energy for its rising population. But concerns have grown because its uranium enrichment program could also make fissile warhead material, because of its nuclear secrecy and also because the Islamic nation refuses to cooperate with attempts to investigate suspicions that it ran experiments related to making nuclear weapons.
While the six want the two-days of talks focused at freezing Iran's uranium enrichment program, Tehran has repeatedly said this activity is not up for discussion. Instead, Iranian officials are pushing an agenda that covers just about everything except its nuclear program: global disarmament, Israel's suspected nuclear arsenal, and Tehran's concerns about U.S. military bases in Iraq and elsewhere.
"We want to discuss the fundamental problems of global politics at Istanbul talks," said Iranian chief negotiator Saeed Jalili. President Mahmoud Ahmadinejad suggested any push to restrict the meeting to Iran's nuclear program would fail.
Tehran is under four sets of U.N. Security Council sanctions for refusing to cease enrichment and other activities that could be used to make nuclear weapons, and Iran came warning it was in no mood to compromise.
"Resolutions, sanctions, threats, computer virus nor even a military attack will stop uranium enrichment in Iran," Ali Asghar Soltanieh, Iran's envoy to the International Atomic Energy Agency, told Iranian state TV.
He was alluding to U.N. sanctions imposed on Iran, apparent damage to the enrichment program due to the Stuxnet malware virus thought to have been created by Israel or the U.S. and threats of possible military action by Israel or the U.S. if Iran remains defiant.
British Foreign Secretary William Hague said the Iranians must "show in these negotiations that they are prepared to discuss the whole of their nuclear program."
EU Foreign Affairs chief Catherine Ashton was expected to urge the Iranian side to recognize the need to discuss international concerns about Iran's nuclear program and perhaps renew a 2008 offer providing Iran with technical support for peaceful nuclear activities as well as trade and other incentives in exchange for focusing on its atomic program.
Diplomats were also watching to see if Jalili would meet U.S. counterpart William Burns in a bilateral meeting something they refused to do in Geneva.
The nuclear talks are being held in the Ciragan Palace, resplendent with marble fittings, balconies and chandeliers, along the Bosporus strait, which divides Istanbul between the Asian and European continents. Fire destroyed the former Ottoman palace in the early 20th century, but the building was restored two decades ago and part of it was turned into a five-star hotel.