With the eyes of the world upon them, the United States and Iran will Thursday have one of their highest-level meetings since the 1979 revolution as their foreign ministers join talks on Tehran's suspect nuclear program.

And while officials are saying that no bilateral talks are planned between US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif, there remains the chance for a quick tete-a-tete in the corridor.

Zarif will be the first Iranian foreign minister to sit down with his counterparts from the five permanent members of the UN Security -- Britain, China, France, Russia and the United States -- plus Germany to discuss Iran's nuclear program.

The Iranian delegation will only join part of the talks being hosted by EU foreign policy chief Catherine Ashton however, and no-one is keen to raise hopes of a breakthrough in the dragging negotiations.

Indeed the encounter with European, Russian and Chinese foreign ministers is set to be brief.

It comes after speculation that Iran's new President Hassan Rouhani would meet or at least shake hands with President Barack Obama at the United Nations fizzled out.

But diplomats say Thursday's meeting will give them the first chance to take the measure of the new Iranian leadership which took office in August.

And they insist it will give the Iranians the opportunity to prove there is some substance behind Rouhani's charm offensive, and his claims that Iran is only seeking to pursue a peaceful civilian nuclear energy program.

Ashton, who has led Western efforts to engage with Tehran, said this week she was "struck by the energy and determination" of Zarif.

But "as you would appreciate, there is a huge amount of work to do," she added.

The international powers made a new proposal to Tehran this year, before Rouhani's election, believed to offer some relief from international sanctions that have crippled Iran's economy in return for a scaling back of its uranium enrichment.

"There's a sense that we never actually got a firm response or a detailed response to that," a senior US official said.

One western diplomat said however that "while we are sensitive to signals there isn't the slightest overture, it's the same old speech."

"We told him that there is an offer on the table, and that if you have an offer we'll look at it carefully," the diplomat said.

"If the Iranians say they want a new round of talks as they have something serious to propose, we'll accept. If there is the slightest opportunity we'll take it."

For his part Zarif said on his Twitter account from New York: "We have a historic opportunity to resolve the nuclear issue," if world powers adjust to the "new Iranian approach."

Obama and Rouhani, in almost back-to-back speeches at the United Nations on Tuesday, both stressed their willingness to try to resolve the nuclear issue. But they came at it from different perspectives.

Rouhani said Iran poses "absolutely no threat to the world," and condemned the international sanctions against his country.

He reiterated Iran's longstanding position that the international community had to accept Iran's nuclear activity, which Western nations say hides an attempt to reach a nuclear bomb capacity.

Noting that Rouhani has said Iran will "never" build a nuclear bomb, Obama said there was a basis for "a meaningful agreement."

But he stressed: "To succeed, conciliatory words will have to be matched by actions that are transparent and verifiable" on the nuclear program.

"The roadblocks may prove to be too great, but I firmly believe the diplomatic path must be tested," Obama said.

And in an interview with The Washington Post, Rouhani said he hoped to reach an agreement on the nuclear program within three months, insisting he had the full backing of the country's Supreme Leader Ali Khamenei to broke a deal.

"The only way forward is for a timeline to be inserted into the negotiations that's short -- and wrap it up. That is a decision of my government, that short is necessary to settle the nuclear file," he said.

"If it's three months that would be Iran's choice, if it's six months that's still good. It's a question of months not years."

In a separate television interview Wednesday, Rouhani said that the right to civilian nuclear energy had become linked to Iranian identity.

"The nuclear issue has turned into an issue of national pride and a symbol of the perseverance of our people," he told "The Charlie Rose Show" on US public television.

Saying that Iranians wanted no more than what is allowed under international law, Rouhani added: "Once we abide by that, I think everything else is settled."