They say that agreement would build confidence in the larger framework of trying to get Iran to discuss concerns about its enrichment program, which can make both fuel and fissile warhead material. The diplomats asked for anonymity in exchange for discussing the closed meeting.

The talks entered their second day Saturday. Iran is under four sets of U.N. Security Council sanctions for refusing to freeze enrichment and other activities that could be used to make weapons. It insists it has no such plans.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

ISTANBUL (AP) — A long day of talks between Iran and six world powers ended with little progress beyond agreement to meet again Saturday, amid stiff resistance by Tehran to demands for discussion of its nuclear activities that could be used to make weapons.

Tehran denies that it wants nuclear arms, 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 Republic refuses to cooperate with U.N. 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.

The length of Friday's negotiations reflected the divide between the two sides with talks lasting for nearly 14 hours before a diplomat familiar with the talks said they recessed for the night. And U.S. comments that focused on what did not happen as opposed to what did, reinforced the lack of progress.

"We would like to see a meaningful and practical negotiation process emerge with Iran's nuclear program as a core focus," said State Department spokesman Philip J. Crowley in Washington. "And, as we have consistently made clear, these meetings are an opportunity for Iran to come forward and address matters that are of great concern to the international community, primarily its nuclear program."

Stating that he could not say whether "the entire issue will be resolved in this meeting in Istanbul, despite our best efforts," Crowley urged reporters to wait for "a fuller report" when the meeting concludes Saturday.

"We're going to wait until the full game has been played, and then we'll report to you as to what has been accomplished and what happens next," he said.

His comments contrasted with Iran's optimistic take earlier in the day, when an envoy from the Islamic Republic said differences were narrowing.

"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."

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 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."

But Iran came to Istanbul 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.

EU foreign affairs chief Catherine Ashton is tasked by the six powers 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 the Iranians refused to do in Geneva. Crowley said that did not happen Friday — despite a series of one-on-ones between the Iranian team and other delegations — but did not say if such an encounter was possible Saturday.

The nuclear talks were 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.