The six powers  the United States and the other four members of the United Nations Security Council, plus Germany  had asked for the extra session in order to shape a framework for further talks and confidence-building measures, the diplomats said. They emphasized that the six would not agree to any preconditions.

After a two-hour opening plenary session on Friday morning largely given over to summarizing known positions, the talks broke at about noon for Friday prayers and lunch, and did not resume again until 4 p.m. Then, a bilateral meeting between the Iranian chief negotiator, Saeed Jalili, and the representative of the world powers, Catherine Ashton, the European Union foreign policy chief, convened for 90 minutes.

But Western diplomats expressed disappointment with the results of the meeting. They said that Ms. Ashton laid out an agreed position on how to move forward, with a revised fuel swap deal for Iran’s already enriched uranium and a series of bilateral meetings, including one with the United States. But Mr. Jalili said that first there must be recognition of Iran’s right to enrich uranium and a lifting of "measures”  understood as sanctions  that would “would jeopardize Iran’s rights and obligations” under the Nuclear Non-Proliferation Treaty.

The diplomats said that Iran’s right to enrich uranium for peaceful purposes was a given, and that the sanctions had been imposed by the United Nations Security Council after Iran failed to deal honestly and completely with the International Atomic Energy Agency. They said they wanted to move into detailed discussions with Iran of a modified fuel swap deal, first laid out in October 2009, designed to remove enough enriched uranium to prevent Iran from building a bomb in return for fuel rods for a small Tehran reactor making medical isotopes.

The point of this Istanbul meeting has been “to find out if Iran is serious” about negotiating, a senior Western diplomat said. There is still no clear answer to that question, but American and French officials in particular have said in the last few days that “talking cannot be an endless process,” even as they have expressed relief that Iran is having difficulties with its centrifuges and the estimated time for it to be nuclear-bomb capable has been extended.