ISTANBUL — Talks between Iran and six world powers about the aims of its nuclear-enrichment program began on Saturday morning with a plenary session of all parties. European and American officials suggested that a serious commitment from Iran to negotiate may be enough to continue the talks at another round in late May, possibly in Baghdad, as Iran has suggested.

“I hope what we will see today is the beginnings of a sustained process,” said Catherine Ashton, the European Union foreign policy chief who is chairing the meeting. The talks, she said, are “to find ways in which we can build confidence between us and ways in which we can demonstrate that Iran is moving away from a nuclear weapons program.” But much depends, she said, “on what Iran is putting on the table today.”

Iran agreed to resume these talks with six major world powers — the five permanent members of the United Nationsl Security Council, plus Germany — after more than a year without any negotiations, raising hopes in the West that Tehran might be ready to strike a deal over its nuclear program, which it denies has any military intent.

The six do not always agree among themselves about tactics, but do want to ensure that Iran will not become a nuclear-weapons-capable state and that it will comply with its requirements under the Non- Proliferation Treaty to open its facilities to complete inspections by the International Atomic Energy Agency.

The diplomats suggested that a positive first step would be for Tehran to agree to allow the inspectors to visit all nuclear sites, including those Iran refused to show them in February. That would help restore confidence and could be enough by itself to open the way to further talks, diplomats said. Iran has fueled Western suspicious by denying the atomic energy agency access to the Parchin military base near Tehran, where the agency says Iran may have tested explosives for warhead research.

If the talks ultimately fail, both the United States and Israel have refused to rule out military action in order to stop Iran’s steady enrichment of uranium to 20 percent purity, just a few technical steps from bomb-grade.

The talks opened informally Friday evening with a dinner between the chief Iranian negotiator, Saeed Jalili, and the chairman of the six powers, Catherine Ashton, the foreign-policy chief of the European Union. The two were to discuss the shape of Saturday’s formal negotiations, diplomats said. Those will open with a plenary session involving all delegates from 10 a.m. until lunch. What happens later on Saturday has not yet been determined, the officials said, suggesting that the plenary session could resume or there could be smaller meetings or bilateral sessions. While the talks are scheduled to end on Saturday, they could go into Sunday, the diplomats said.

On Friday, Iran’s deputy negotiator, Ali Baqeri, held separate talks with senior Chinese and Russian officials in Istanbul, and the six powers met among themselves to coordinate tactics. Russia and China have been the most reluctant of the six to press for further sanctions on Iran, and Russia’s chief negotiator, Sergei Rybakov, deputy foreign minister, has urged a more moderate tone with Iran.

“We really need to find a middle course,” he told reporters on Friday. “The negotiations are about renewing confidence” and “the final destination in the near future” of the talks should be to remove sanctions against Iran.

Western diplomats have expressed cautious optimism that Iran, struggling under the vise of new sanctions on its oil exports and central bank, is ready to discuss limits on its nuclear program, especially on its enrichment to 20 percent. But the diplomats also cautioned against any specific deal here that would produce a lifting of any sanctions, other than an agreement that the talks are serious and substantive enough to continue them.

“I don’t think they would come if they weren’t serious,” one Western diplomat said, adding that a second meeting could take place by the end of May.

But Iran’s domestic media cited sources close to Iran’s delegation as saying Tehran saw “few encouraging points” in the remarks of American and European officials. In Washington on Thursday, G-8 foreign ministers said in a statement: “Iran’s persistent failure to comply with its obligations... and to meet the requirements of the IAEA Board of Governors resolutions is a cause of urgent concern.”

Iran says it is coming with new proposals of its own, and says it will not allow any preconditions for the talks.

Ben Rhodes, the deputy White House national security adviser, said on Air Force One that Washington wanted a “positive environment” in the talks with Tehran showing “seriousness” about moving forward with dialogue, news agencies reported.

“I think nobody expects to resolve all differences in one meeting, but what we want is a positive environment where the Iranian government demonstrates its seriousness and its commitment to pursuing serious negotiations,” he said. The Iranians needed to show how they could “build confidence” with the international community by living up to commitments to disclose details of what the West believes is a nuclear weapons program. “If they do, we would certainly explore reciprocal actions that are responsive to concrete steps by the Iranians,” Mr. Rhodes said.

Washington is eager to take some of the urgency out of the Iranian confrontation and to reassure Israel that there is time for diplomacy and sanctions to work. In particular, in the next few months, Washington would like to ensure that Iran stops enriching uranium to 20 percent, ships out its existing stocks and shuts down or downgrades its enrichment site at Fordo, which, because it is built inside a mountain, would be difficult to bomb.

Iran’s supreme leader, Ayatollah Ali Khamenei, is the key figure behind the scenes and authorized Mr. Jalili to resume the talks as his representative, a significant signal to the West. Ayatollah Khamenei has recently repeated his fatwa against acquiring nuclear weapons, saying that they are against Islam, which some Western experts see as a way to prepare the Iranian people for any concessions Tehran makes in these talks and those that may follow.

The fatwa, however, does not address the issue of nuclear capability and could be changed if the context changes — much as a legal decision might. (Copyright 2012 The New York Times Company 04/14/12)