As Iran's new president reaches out to the West with a message of moderation, Israel's hawkish approach on Tehran's nuclear programme looks likely to come under pressure, experts say.

President Hassan Rouhani on Tuesday emphasised the peaceful nature of the Islamic republic's atomic programme, telling the UN General Assembly that "nuclear weapons... have no place in Iran's security and defence doctrine."

Israel scrambled to denounce the speech, with Prime Minister Benjamin Netanyahu calling it "cynical" and "full of hypocrisy".

"This is exactly Iran's strategy -- to talk and play for time in order to advance its ability to achieve nuclear weapons. Rouhani knows this well," charged Netanyahu.

But Rouhani's diplomatic overtures, which stand in stark contrast to the belligerent statements so commonly heard from his predecessor Mahmoud Ahmadinejad, are proving to be quite a challenge to the Israeli premier.

"For the past eight years, Israel's efforts to convince the world... to tackle Iran's nuclear designs head on relied on... adamant, Holocaust-denying Ahmadinejad," commentator Chemi Shalevan wrote in Haaretz newspaper.

"Ahmadinejad... served as Israel's number one talking point, its strategic propaganda asset, a poster boy who self-explained Tehran's sinister designs."

Rouhani's message is a "real diplomatic challenge for Israel," Professor Uzi Rabi, an Iran specialist at Tel Aviv University, told AFP.

"The last time Netanyahu was at the United Nations, it was much easier for Israel to argue on Iran," he said.

Last year, Netanyahu stood before the UN General Assembly and drew a red line on a cartoonish depiction of a bomb, saying the international community must act to prevent Iran from using its nuclear programme to build a weapon, a charge accepted by the West but denied by Tehran.

This year, when he addresses the UN General Assembly on October 1, the Israeli leader will seek to play down the differences between Rouhani and his predecessor, the top-selling Yediot Aharonot daily said.

"Netanyahu will say that, like Ahmadinejad, Rouhani also adheres to the goal of destroying the State of Israel and attacking the entire Western world."

Rabi agreed: "Israel is trying to make sure everyone is keenly aware that the Iranian charm offensive is just tactics - it doesn't mean there's real change."

Israel's concerns over a thaw were further stoked on Monday when officials said US Secretary of State John Kerry would hold his first nuclear talks with Iran's Foreign Minister Mohammad Javad Zarif at a landmark meeting at the UN headquarters on September 26.

"Netanyahu... is not going to be in the negotiations room so he's trying to remind the US of the reality that there's not been evidence of change," Dr Emily Landau of Tel Aviv University's Institute for National Security Studies told AFP.

Israel fears the meeting could undermine its efforts to put further pressure on Iran, according to Dr Raz Zimmt, a research fellow at Tel Aviv University's Iranian studies centre.

"Israel's government and prime minister are facing difficulties convincing the international community to treat Iran the same way it has treated it before," Zimmt told AFP.

"In the West, they see Rouhani as totally different from Ahmadinejad. They're right, but... Rouhani hasn't come up with a concrete proposal on how to resolve the nuclear issue.

Washington has been "working behind the scenes to allay Israeli concerns and has said that the Iranian president will be judged by his actions," Israel's Maariv newspaper said on Sunday.

But Rouhani's "charm offensive," together with developments in the wider Middle East region, may trump Israel's protestations, analysts warned.

"Both the US and Iranian administrations have decided there's a window of opportunity to talk," Zimmt said.

Holocaust is 'reprehensible'

In a radical about-turn from his Holocaust-denying predecessor's rhetoric, Rouhani, in an interview with CNN aired on Tuesday, condemned the Nazi genocide as a "reprehensible" crime.

"I'm not sure the US can help it. Rouhani is the star of the UN," Rabi said.

"The Iranians could come up with an interim suspension of uranium enrichment, but in the long term Iran is going to prevail when it comes to its nuclear plan," he predicted.

And with much of the Middle East in crisis, Iran knows Washington may need its help in other areas, such as Syria.

"This is why they're coming up with a new initiative" to talk with the West, Rabi told AFP, saying Iran was "capitalising on the Syria saga."

In a bitingly sarcastic tweet, the Israeli embassy in Washington, has derided Iran's new president as an expert salesman who is bullish on nuclear proliferation.

The tweet poses the question of what Rouhani's LinkedIn page might look like.

The spoof of his LinkedIn page has him describing himself as "a career politician, expert public relations professional, leading international salesman and longtime advocate of nuclear proliferation."