With each week that passes, Iran's ayatollahs move closer to their goal of building an atom bomb. This is not misinformed propaganda pumped out by trigger-happy yahoos on the wilder fringes of America's Republican Party. This is the opinion of the dedicated teams of nuclear experts attached to the International Atomic Energy Agency (IAEA) in Vienna, whose task it is to sift through the highly complex science surrounding Iran's nuclear programme and to provide a considered judgment to the UN Security Council on the Iranians' ultimate objectives.

During three years of painstaking negotiations with Iran, Mohamed ElBaradei, the Nobel peace laureate who heads the IAEA, went out of his way to play along with the charade that Iran's nuclear ambitions were entirely peaceful and designed to develop an indigenous nuclear power industry. This, after all, is a country with known oil reserves in excess of 90 billion barrels, more than enough to meet its energy needs well into the next century.

Mr ElBaradei was even prepared to accept at face value the Iranians' shame-faced admission that their failure to disclose the existence of their massive nuclear enrichment plant at Natanz was no more than a bureaucratic oversight. When the inspectors were finally granted admission, they were dumb-founded to find themselves in a 250,000-acre complex containing two vast underground bomb-proof bunkers designed for enriching uranium to weapons grade.

Mr ElBaradei is now prepared to concede that the Iranians have run out of excuses, and Teheran has been given until April 29 to implement a total freeze on its nuclear enrichment activities at Natanz and its other key plants, or face the wrath of the Security Council. At the same time the IAEA's nuclear specialists are working on a report that will be submitted to the UN on the same day, in which they will state explicitly their concerns about Iran's nuclear programme.

But to judge by the Iranians' response so far, the threat of international condemnation and isolation does not appear to be causing sleepless nights.

This is because, while Western diplomats agonise over how to deal with the threat posed by Iran's nuclear programme, Iranian scientists are working hard to achieve nuclear enrichment, processing uranium to a level where it can be used to make atomic weapons.

Far from taking the UN's ultimatum seriously, nuclear experts at the IAEA now report that Iranian scientists at Natanz are taking advantage of the diplomatic stand-off to intensify their efforts to develop the technical capability to enrich uranium to weapons- grade. This process began in January, when they began assembling new centrifuges, the sophisticated equipment needed to enrich high- grade uranium. Their ambition is to link 164 centrifuges, thereby forming a "cascade''. Once that is accomplished, Iran will be able to produce its own weapons-grade uranium.

Estimates vary as to how long it will take the Iranians to accomplish such a technically demanding task, and how long it will then take them to make an atom bomb. The hawks argue that Iran could have enough material for a nuclear bomb within three years, while the more sanguine members of the international intelligence community say it could take 10 years.

What is not in doubt is that the work now being undertaken at Natanz, and at the processing plant at Isfahan, means the Iranians will soon be self-sufficient in producing weapons-grade uranium. And once they have passed that important milestone, it is then merely a question of when, not if, they develop a nuclear arsenal.

"Iran's strategy all along has been to talk and at the same time proceed with its nuclear programme,'' said an official closely involved in the IAEA's negotiations with Iran. "The longer we draw out the diplomatic process, the closer they get to fulfilling their nuclear ambitions.''

The mounting frustration, particularly within the Bush Administration, over the UN's impotence to prevent Iran fulfilling its nuclear destiny explains the recent hysterical reports suggesting that George W. Bush is seriously contemplating nuclear air strikes against Iran's bomb-making infrastructure.

It is no coincidence that these reports are circulating at a time when the Iranians themselves are indulging in their own sabre- rattling, with their armed forces undertaking a series of military exercises in which they are showing off all their latest technological advances, from radar-evading missiles to stealth flying boats.

While none of these weapons would seriously threaten the overwhelming superiority enjoyed by America, the clear signal that the Iranians are trying to send out is that, if attacked, they have the ability to retaliate and cause mayhem throughout the Middle East. Apart from starving the West of vital oil supplies by closing the Straits of Hormuz, the Iranians have an advanced ballistic missile capability that can hit targets throughout the Middle East - including Israel.

Certainly the fear of provoking a wider Middle East war is one of the reasons that divisions are already starting to appear in the Security Council over how best to deal with Iran. While Britain and America would like to see a "smart sanctions'' regime implemented if Teheran refuses to call a halt to its nuclear enrichment activities by the end of this month, other powerful voices, particularly Russia and China, believe such a move would be counter-productive.

Irrespective of the outcome, however, the Bush Administration is correct in its assessment that, without the threat of serious military action, the Iranians are unlikely to take seriously the West's determination to prevent them acquiring a nuclear arsenal.

The suggestion, contained in Seymour Hersh's article in this week's New Yorker, that Washington is prepared to use tactical nuclear weapons, might appear far-fetched: the ground-penetrating bombs used to destroy Saddam's state-of-the art German-built bunkers at the start of the Iraq war three years ago adequately accomplished the task using conventional munitions.

But if the current round of diplomacy is to stand any chance of success, then the Iranians must be made to understand that their prevarication tactics at the UN can no longer be tolerated over an issue of such importance for international security. For while Jack Straw, the Foreign Secretary, regards the concept of military action against Iran as "nuts'', it would be even nuttier to allow Teheran to have an atom bomb.