Updated

As diplomats prepare for the annual whirlwind of formal talks and private side meetings at next week’s gathering of the United Nations General Assembly in New York, the Obama administration and its overseas allies are debating how best to respond to the seemingly friendlier face – and negotiating posture – of Iran.

Recent weeks have seen the newly inaugurated Iranian president, Hassan Rouhani, launch what passes, for the Islamic regime, as a charm offensive.

Rouhani has used Twitter to post a Rosh Hashanah greeting to Jews around the world – an astonishing move after eight years of inflammatory Holocaust denial by his predecessor, Mahmoud Ahmadinejad. He has released jailed political prisoners. He has exchanged courteous letters with President Obama.

Rouhani also has hinted at a new flexibility in the longstanding impasse over Tehran’s march toward a nuclear weapons capability, which has prompted Western allies to impose ever-tougher economic sanctions on the regime.

But European diplomats, speaking to Fox News on condition of anonymity so as not to prejudice the allies’ diplomacy, say they do not believe Rouhani’s claim that he enjoys “full authority” to negotiate a resolution to the nuclear issue, as he told NBC News’ Ann Curry on Wednesday in his first interview with a Western journalist. That primacy in Iranian policymaking, officials remain convinced, resides with the country’s supreme leader, the Ayatollah Ali Khameini.

“But that does not mean he has no authority,” said one diplomat, representing one of the P5 + 1 nations, the group of five U.N. Security Council member states (the U.S., Britain, France, Russia and China) plus Germany that conducts nuclear diplomacy with Iran.

However, officials at European foreign ministries say they have had a difficult time getting the U.S. to focus on the Iranian account, given the Obama administration’s recent preoccupation with dismantling the Syrian chemical weapons arsenal, reviving the moribund Israeli-Palestinian peace process, and responding to the military coup that ousted the Islamist leader Mohamed Morsi from power in Egypt this past summer.

Among the considerations American and allied diplomats are weighing now is whether to respond to Rouhani’s limited but highly symbolic overtures with some immediate, and similarly limited, reciprocal measures, which would be aimed at boosting Rouhani’s internal standing with Iranian hardliners and encouraging him to continue on his current path of conciliation.

Such measures, however, would not include the easing or lifting of the economic sanctions, diplomats said, because none of the overtures from Tehran thus far have been directly related to the nuclear work the regime continues to perform – and the August 28 report of the International Atomic Energy Agency (IAEA), the United Nations’ nuclear watchdog, estimates Iran produced nearly 50 kilograms of highly-enriched uranium, suitable for use in a nuclear weapons, in the preceding 90 days.

Nuclear talks will resume in earnest when the French foreign minister, Laurent Fabius, meets directly with his Iranian counterpart on the sidelines of the U.N. General Assembly sessions in New York next week.

By then, the allies hope to have agreed, at least privately, on a new package to offer Tehran. This would replace the incentives offered at the last round of meetings with the Iranians, held in Kazakhstan last February, and could conceivably be more appealing to the regime.

The possible menu items include offering Tehran a sped-up timetable of action for action, step for step. Diplomats said the actions would include confidence-building measures from the Iranians, such as the suspension of uranium enrichment, or the agreement not to enrich to certain levels, or the readmission of IAEA monitors to specific sites, in exchange for the easing of the economic sanctions.

“The new president has made clear that he wants to -- or says that he wants to -- address that problem,” White House spokesman Jay Carney said Thursday. “And to do that, he needs to demonstrate that he's serious about resolving this conflict with the international community.”

Secretary of State John Kerry told reporters a short while later that Rouhani’s comments to date have been “very positive,” but added, “Everything needs to be put to the test.”

Veteran Iran watchers caution that previous outreach campaigns by new Iranian leaders have seldom led to substantive progress on the nuclear account – and also that American policymakers have compiled a spotty record, across several administrations, in their efforts to understand and affect Iranian decision-making.

“While they are determined to obtain a nuclear weapons capability and they've paid a high price for this in recent years with the sanctions, they're also willing to be flexible when we show our teeth,” said James Jeffrey, a former deputy national security adviser to President George W. Bush and now a scholar at the Washington Institute for Near East Policy.

“That's what happened in 2003, when they suspended enrichment. That has happened at other times. So the speed at which they will move to that – and whether they will actually take the final step – is basically dependent upon the right set of American and international actions against Iran.”

Kenneth Pollack, a senior fellow at the Saban Center for Middle East Policy in Washington, is the author of a new book, entitled “Unthinkable: Iran, the Bomb, and American Strategy,” that argues it would be less costly for the United States to seek to contain a nuclear-armed Iran than to encourage, or engage in, military action to prevent Iran from going nuclear.

“Iran's president isn't unimportant. He has a role to play in the Iranian government and he certainly has influence over Iranian policies. But we shouldn't assume that Iran's president is the ultimate decision-maker in Iran. He isn't,” Pollack told Fox News.

“At the end of the day, it's all about the supreme leader, Ayatollah Ali Khamenei. And what Ayatollah Ali Khamenei thinks is often the most difficult thing to gauge.”