Middle East meets west: Iranian President Hassan Rouhani greets French President Francois Hollande at the UN. Photograph: Craig Ruttle/AP  The smile offensive conducted by Iran's President Hassan Rouhani at the UN has not been greeted with smiles in Israel. While welcoming a "genuine diplomatic solution" to the Iranian nuclear challenge, the prime minister, Binyamin Netanyahu, this week warned the world not to be "fooled by half-measures that merely provide a smokescreen for Iran's continual pursuit of nuclear weapons". Iran's history of deceit on this issue calls for rephrasing President Reagan's famous reference to the Soviet Union: "Do not trust but verify." Give Rouhani a chance, but test him by his deeds. Israel neither trusts Rouhani nor fully relies on President Obama's resolve.  Speaking at the UN general assembly last year, Netanyahu drew a red line at Iranian production of one bomb's worth of 20% enriched uranium. In the past year, however, advances in the Iranian programme have made this red line irrelevant. In addition, Rouhani was voted in offering another chance for diplomacy, while Obama equivocated on his own red line regarding the use of chemical weapons in Syria. From Israel's perspective, spinning open-ended diplomacy while centrifuges continue to spin – and while there is no credible military deterrent – is a dangerous situation.  When nearing the line with its 20% enrichment, Iran converted most of the excess amounts to oxide form, which could be reprocessed back in a matter of weeks. At the same time, it added many centrifuges and is installing a new centrifuge generation (IR-2m), about four times faster than the old type. With its expanding arsenal and with the more advanced centrifuges, Iran may be able in the coming year to break out to a bomb's worth of enriched uranium (over 90%) within several weeks, and to a nuclear device within a few months, starting with uranium enriched only to 3.5%.  Looming over the horizon is an even more dangerous plutonium programme: a heavy-water reactor is under construction at Arak. Once this reactor becomes "hot" it may be impossible to stop through military means, for fear of radiation leakage.  The diplomatic option is further complicated by the fact that it hitherto focused on 20% enriched uranium. The P5+1 (United States, Russia, China, United Kingdom, France and Germany) demanded that Iran cease enrichment to this level, ship out enriched material and close the enrichment facility at Fordow, which is devoted to enriching to 20% and seemingly invulnerable to air strikes. Yet it failed to address other developing dimensions of the Iranian programme, which could soon afford Iran the capacity to breakout to a bomb before being stopped. These include the number of centrifuges, their quality, weaponisation designs and the plutonium track.  Enter Rouhani. With his charm offensive, he provides relief from the confrontational style of his predecessor. Yet in the rough terrain of the Middle East, where suspicion is a common trait hardened by long experience, there is ample reason to suspect his motives. Rouhani is the flesh and blood of an Iranian system which has heavily invested in its nuclear programme and is unlikely to give up on it. And at the end of the day, he answers to the supreme leader, Ali Hosseini Khamenai. His recent expressed interest in a quick diplomatic deal is tempered by insistence on continued enrichment. He has boasted on the record of how he had managed, as the regime's nuclear negotiator a decade ago, to move the programme forward while engaging the west and averting sanctions. Are we facing a replay?  With international sanctions proving crippling – and driving Iran to seek a diplomatic exit – it is important not to relax them before the regime proves it has really changed course, or else it will be impossible to step the pressure up again. It is equally important not to allow this diplomatic endeavour to drag on endlessly while Iran develops its programme to a critical breakout capacity.  Israelis also believe that for diplomacy to work it must be backed by credible military deterrence. It may be that Syria's President Assad was impressed by the US military threat and so opted for a diplomatic deal. In Israel, however, Obama's obvious recoiling from enforcing his red line raises questions over his resolve to carry the burdens of prevention in the face of Iran.  This all contributes to a gloomy feeling in Israel that if diplomacy fails, the country will be left alone with a terrible decision between "the bomb" and "the bombing". If diplomacy yields a bad deal, which does not sufficiently set Iran back from breakout capacity, Israel's dilemma will be no less daunting.  • Comments on this article are set to remain on for 24 hours but may be closed overnight
Michael Herzog: The peace overtures in New York have not been echoed in Tel Aviv, where the mood is one of isolation and gloom