ISTANBUL — At negotiations this week between Iran and six world powers, the United States and its allies hope to make enough progress to take some of the urgency out of the confrontation over Tehran’s nuclear program, to reassure Israel and to arrange a second round of talks soon.

For the first time in years, both Iran and the six powers — the United States, Russia, China, France, Britain and Germany — seem interested in serious negotiations that both sides have agreed will take a “step by step” approach and be “reciprocal.” And both sides say they are coming to the talks here in Istanbul with proposals on the nuclear issue in discussions that may begin over dinner on Friday night and will continue formally on Saturday. Iranian state media said Tehran’s delegation arrived in Istanbul on Friday morning.

But considerable doubts remain about how far Iran is willing to go to satisfy the United Nations Security Council’s demands to suspend uranium enrichment and cooperate with the International Atomic Energy Agency. One reason is the West’s experience with Iran’s lead negotiator, Saeed Jalili, who is considered tough and inflexible, is given to long speeches and is careful to stick to his negotiating instructions.

Mr. Jalili, 46, an engineer by training, is the secretary of Iran’s Supreme National Security Council. His career has had its ups and downs, but it appears to be on the rise again.

Intriguingly, diplomats said, Mr. Jalili has acquired an additional title in his correspondence with the representative of the six powers, Catherine Ashton, the European Union’s foreign policy chief. Mr. Jalili’s letterhead identifies him as the “personal representative of the supreme leader,” Ayatollah Ali Khamenei, presumably meaning that Mr. Jalili will speak on his behalf in Istanbul, at least up to a point. That alone would be a major change, the diplomats said, especially in an Iranian system that everyone outside of it agrees is opaque.

“When the Iranians do things like this we think it means something,” said Volker Perthes, director of Germany’s Institute for International and Security Affairs. “Jalili seems to be indicating that you can do business with me because the supreme leader is behind my mission. That’s a change.”

At the last round of these talks, in Istanbul in January 2011, Mr. Jalili lectured rather than negotiated, and demanded preconditions for serious talks that were unacceptable to the six powers. But Mr. Jalili was also Iran’s lead negotiator in Geneva in October 2009, when a tentative agreement was reached to export most of Iran’s enriched uranium in return for nuclear fuel. That deal, openly supported by President Mahmoud Ahmadinejad of Iran, was rejected by Ayatollah Khamenei.

Both Mr. Jalili and Mr. Ahmadinejad suffered when that deal fell apart, derailed partly by a fierce presidential election in Iran and by the Iranian hierarchy’s belief that President Obama was not serious about the talks, said Farideh Farhi, an independent scholar and expert in Iranian politics at the University of Hawaii at Manoa.

But the context is different now. Iran has produced a much greater quantity of enriched uranium, some of it at 20 percent purity, just a few technical steps from bomb grade; it has placed many more centrifuges deep inside a protected mountain at Fordo, near Qum; and it is facing increased sanctions that are causing severe economic distress. So there is more of a sense of urgency on both sides, with Israeli leaders talking openly of bombing Iran’s nuclear sites before it becomes too difficult to do so.

There are important internal changes in Iran, too. Iran appears to have suppressed its “green revolution,” while Ayatollah Khamenei has solidified his power and Mr. Ahmadinejad, who had challenged him, is considered much weakened. Ayatollah Khamenei has been especially protective of Iran’s nuclear program, while at the same time saying that it is against Islam to possess nuclear weapons. Iran denies that its nuclear program has a military aim, but most members of the Security Council and the International Atomic Energy Agency have expressed doubts.

The United States has made clear that the 20-percent-enriched uranium and the protected Fordo site are matters of urgency, and Iran has hinted that it may be willing to suspend enrichment to 20 percent, at least temporarily, in return for concessions — which may include, for instance, suspending the European Union’s oil embargo, scheduled to begin July 1, or even of some sanctions against Iran’s Central Bank. Reciprocal suspensions of some kind, experts say, might be enough to buy time and get both sides to another round of talks.

“Jalili is not considered to be very smart, even inside Iran,” Ms. Farhi said. “He used to work in the office of Khamenei, and wouldn’t go to Istanbul without specific instructions about how serious the talks should be. But for Iran, some things have changed: sanctions have become harsher, and they believe the kinds of technological advances they pushed through have made a difference in American calculations.”

So this time “both sides seem a bit more optimistic, and both sides are worried about failure,” Ms. Farhi said. “There is more of a balance of interests; that brings a seriousness to this discussion.”

Secretary of State Hillary Rodham Clinton, talking to reporters in Washington on Thursday about the impending talks, acknowledged that the Iranians had signaled their intent to bring up new ideas, and she suggested that the United States was prepared to consider reciprocal steps. But Mrs. Clinton also said the Iranians must “demonstrate clearly in the actions they propose that they have truly abandoned any nuclear weapons ambition.”

Mark Fitzpatrick, a nuclear expert at the International Institute for Strategic Studies and a former senior official at the State Department, said he also saw a new impetus for these talks stemming from the pain of the economic sanctions on Iran and of Washington’s “keen interest in persuading Iran to accept some limits” so that Israeli military action can be postponed and talks can continue.

He also said that Mr. Jalili, humiliated by the rejection of the 2009 deal, clearly had moved closer to Ayatollah Khamenei and had been an important figure in Iran’s rejection in February of closer cooperation with the international atomic agency on military inspections in Iran. Those inspections were meant to satisfy lingering questions about Iran’s nuclear intentions.

“So he’s seen as a hard-liner,” Mr. Fitzpatrick said. “And he’s again the one fronting for Iran, so expectations are not high that he of his own volition would be looking for a way out.”

In the end, all agree, any key decisions will rest with Ayatollah Khamenei, 72, who since he became supreme leader in 1989 has remained highly suspicious of the United States.

Karim Sadjadpour, an Iran expert at the Carnegie Endowment of International Peace in Washington, said that “in past meetings, Jalili has been less a negotiator and more a human tape recorder who simply repeats official government slogans.” But Iran will have to make “meaningful compromises,” he said, if it wants to avert new sanctions and participate in a further round of talks.

Mr. Sadjadpour expressed doubts that Ayatollah Khamenei is “capable of suddenly reinventing himself at age 72.” No deal can be made without him, “but what’s unclear is if any deal can be made with him.”

Steven Lee Myers contributed reporting from Washington. (Copyright 2012 The New York Times Company 04/13/12)