In this 2013 photo released by the official website of the office of the Iranian Presidency, Iranian President Hassan Rouhani speaks during an interview with state television in Tehran. Rouhani is set to address the U.N. on Tuesday. (Rouzbeh Jadidoleslam/AP)

In the often colorless realm of international diplomacy, one annual ritual has stood out for years — a provocative speech by Iran’s leader to the United Nations each September, followed by a walkout by U.S. and Israeli delegates.

That bit of diplomatic theater may not happen this year, as a new Iranian president who has alternately pleased, intrigued and startled American observers makes his debut trip to the annual gathering of world leaders in New York.

No one, including Obama administration officials, knows what might happen instead.

Iranian President Hassan Rouhani and President Obama are set to address the world body on Tuesday, with the tantalizing prospect of a face-to-face meeting between the leaders of two nations so long poised as enemies.

Rouhani’s speech is expected to be the main event in nearly a week of interviews, think-tank talks and other appearances meant to showcase a newly moderate, approachable face of the Iranian government. The charm offensive included an op-ed by the Iranian leader in Friday’s Washington Post.

“As I depart for New York for the opening of the U.N. General Assembly, I urge my counterparts to seize the opportunity presented by Iran’s recent election,” Rouhani wrote in the op-ed. “I urge them to make the most of the mandate for prudent engagement that my people have given me and to respond genuinely to my government’s efforts to engage in constructive dialogue.”

U.S. officials say they have been encouraged by recent gestures from Rouhani, including the public discussion of his country’s disputed nuclear program — the crux of Iran’s standoff with the West. Obama has said he will do whatever it takes, including a military strike, to stop Iran from building a bomb. Rouhani said in a television interview this week that his country will never seek nuclear weapons.

“There’s no question that the new Iranian government has been taking a different approach in the things that it has said about a lot of issues,” White House press secretary Jay Carney said Thursday. “It has taken some actions that suggest a new approach.”

Carney and other U.S. officials, however, have been careful not to embrace Rouhani without reservations and have emphasized that Iran’s stated desire to improve relations with the international community needs to be followed up with concrete steps.

“Rouhani’s comments are very positive, but everything needs to be put to the test, and we’ll see where we go,” Secretary of State John F. Kerry said Thursday.

A contrast to Ahmadinejad

The barrage of friendly Iranian gestures marks a U-turn away from the rhetoric of Rouhani’s immediate predecessor, Mahmoud Ahmadinejad, who for a decade regularly denounced the United States and Israel and suggested that the Holocaust was a myth.

If Obama is to make good on his 2008 campaign promise to explore the possibility of improved relations and a nuclear rapprochement with Iran, the U.N. meeting this year appears to be his best chance yet.

Rouhani stunned many with a Rosh Hashanah greeting to Jews this month and announced Thursday that he will bring Iran’s only Jewish member of parliament with him to New York.

This week, Iran’s supreme leader, Ayatollah Ali Khamenei, signaled a willingness to begin negotiations on the nuclear issue. Iran also released several high-profile political prisoners.

Carney said there is no meeting scheduled between Rouhani and Obama. But Rouhani has suggested that he is open to a meeting, which would be the first such contact since 1979, when the countries broke off relations after Iranian students stormed the U.S. Embassy in Tehran and held 52 Americans hostage for 444 days.

Rouhani told NBC News that he welcomed a letter of congratulations that Obama sent him last month. “From my point of view, the tone of the letter was positive and constructive,” Rouhani said through an interpreter.

Since taking office last month, Rouhani and his U.S.-educated foreign minister, Mohammad Javad Zarif, have offered a unified vision of Iran’s foreign policy agenda. That is in stark contrast to the hodgepodge of the Ahmadinejad years, when different centers of power competed for influence.

Rouhani also appears to have the support of Iran’s supreme leader, who has long been the ultimate arbiter of the country’s nuclear decisions.

That endorsement brings an air of authority to Rouhani’s calls for increased openness toward the West, said Dennis Ross, a former senior adviser on the Middle East to the Obama White House.

“The fact that the Iranians, including the supreme leader, are publicly indicating a readiness to talk means they are preparing the ground for such engagement,” Ross said.

High stakes for Iran, West

Still, there is more at stake now, with Iran closer to the point at which it could build a bomb if it chose to do so and its economy wobbling under the weight of international sanctions imposed because of the nuclear dispute.

Veterans of past negotiations with Iran said they fear that Rouhani’s team may not yet grasp the kinds of concessions that the Islamic republic would be required to make to win relief from the most painful economic sanctions, such as restrictions on banking transactions and oil sales. Iranian officials have hinted at a willingness to accept minor limits on their nuclear program, which would not prevent a quick drive for a bomb.

“Charm alone is insufficient to achieve the kind of sanctions relief they’re looking for,” said Gary Samore, who until early this year was Obama’s principal adviser on arms control and weapons of mass destruction. “So far I haven’t seen the Iranians indicating a willingness to take the dramatic steps necessary to lift the biggest sanctions.”

Zarif, who is well known to U.S. officials as Iran’s ambassador to the United Nations from 2002 to 2007, is already in New York. He hosted a closed-door luncheon for more than 100 U.N. diplomats on Thursday. A participant, speaking on the condition of anonymity to detail the private session, said Zarif welcomed signs that “drums of war in the region” were going silent and vowed to resolve concerns about Iran’s nuclear program.

Kerry and Zarif may meet on the sidelines of the U.N. session, though neither side has announced any specifics.

Samore said the best hope for progress in New York this year could be establishing a forum for direct, if low-level, talks between the U.S. and Iranian governments, building a foundation for a future nuclear accord.

Some of the proposals floated in recent days suggest that Iran is setting the stage for a possible deal with the bloc of six nations that has engaged in intermittent negotiations with the country over several years. But Western diplomats and policy experts say the issues in contention are probably too complex for meetings on the sidelines of the U.N. conference.

For now, Iranian hard-liners who have championed their country’s nuclear capability appear to be sidelined, said Mohammad Ali Shabani, a Tehran political analyst. If Rouhani’s overtures yield nothing, Shabani said, there will be a political and public backlash.

“Right now, this government’s hands aren’t tied, but that won’t last forever. Right now, hands are open,” he said.

Rezaian reported from Tehran. Colum Lynch at the United Nations and Joby Warrick in Washington contributed to this report.