As pressure mounts for new sanctions against Iran, experts say its alleged nuclear weapons programme is struggling to find scientists and technicians and faces sabotage by Western and Israeli agents.

Despite already being the subject of economic sanctions and facing the threat of more concerted international action, Iran is pushing ahead with attempts to enrich large quantities of uranium to make nuclear fuel.

Having said it plans to refine its three or four percent enriched uranium to the 20 percent level that could be used in a research reactor, Tehran is on the threshold of perfecting the 90 percent strength needed for a bomb.

"It's going slower ... than they anticipated. But they are moving in that direction," US Defence Secretary Robert Gates said on Sunday in an interview with the NBC network.

Once 90 percent enrichment is achieved, it will only remain for Iran to assemble a missile with separate stages, something intelligence agents think Iran has made considerable progress towards over the past three years.

China was continuing Tuesday to resist Western calls for new economic sanctions, disappointing Washington by calling for "dialogue and negotiation" just ahead of a 47-nation summit on world nuclear proliferation.

But in the meantime, Iran has already run into technical difficulties, some related to its limited engineering capacity, others deliberately engineered by Western intelligence in a bid to sabotage the programme.

"The Iranians are still enriching uranium, but they've run into some problems," a senior European official told AFP, speaking on condition of anonymity to confirm details of a covert international operation.

According to Western experts, Iran's array of centrifuges has allowed it to produce around two tonnes of weakly enriched uranium, but only at the cost of working its out-of-date equipment almost to breaking point.

In a race to beat imminent sanctions and the possible threat of Israeli or US air strikes, Iranian scientists rely on so-called "dual use" imported civilian equipment to maintain their creaking production line.

This is where Western spy networks come in.

Having identified much of Iran's international network of buyers and front organisations, legally and illegally gathering the tools Tehran needs, Western agents are able to almost-literally throw a spanner in the works.

"For example, if a certain kind of lubricant is needed for such-and-such a task, it's delivered to them. Then, six months later, it starts to corrode the material it has been applied to," one specialist told AFP.

Jean-Pierre Maulny, an analyst at the Institute of International and Strategic Relations in Paris, confirmed the practice, which he said was possible thanks to what is known as the "Wassenaar Arrangement".

Set up in Vienna in 1995 and with 35 members, the Wassenaar group shares information on what other countries are seeking to import in terms of weapons and civilian equipment with possible military applications.

Once a picture is built up of a possible rogue state's intentions, other agencies -- such as national intelligence services -- can act.

Where are Iran's centrifuges built? Where are those that were to be installed in its recently revealed second enrichment centre outside Qom? Are there other secret sites in Iran? What can be done about them?

Spy agencies want answers to all these questions, and their task could be made easier by the other weak link in Tehran's enrichment chain: its expert personnel, from skilled welders to top-flight nuclear scientists.

In January, Massoud Ali Mohammadi, 50-year-old Iranian nuclear physicist, was killed by a remote-controlled bomb hidden on a motorcycle parked near his Tehran home.

Iran has blamed the attack on either the US Central Intelligence Agency (CIA) or Israel's Mossad.

Another Iranian physicist, Shahram Amiri, disappeared in June last year on pilgrimage to Saudi Arabia. US media report that he willingly defected to the CIA. Iran claims he was kidnapped.

Either way, he is no longer available to help build an Iranian bomb.