Tehran has 'technical ability' to make highly enriched uranium, say experts, as efforts turn to disrupting supply of other materials

US officials believe Iran now has the "technical ability" to make highly enriched uranium, an essential step towards building a nuclear bomb, according to a leaked US diplomatic cable.

At a meeting of international nuclear experts in Vienna, a US representative said that "Iran had now demonstrated centrifuge operations such that it had the technical ability to produce highly enriched uranium (HEU) if it so chose". The meeting in April 2009 was hosted by Britain's ambassador to Austria, Simon Smith.

The revelation comes as nuclear talks resume between Iranian officials and representatives of six world powers in Istanbul tomorrow. Expectations of compromise are low.

Not all western governments share the US conclusion, but if true it suggests international sanctions have failed to deny Iran the know-how required to make a nuclear bomb. The production of HEU is generally agreed to be the most serious obstacle any aspiring nuclear state must overcome.

Diplomatic efforts to persuade Iran to curb its programme have also failed to produce results. In the days running up to the two-day meeting in Istanbul, Iranian officials have repeated they will not bow to UN security council demands to suspend uranium enrichment. Iran insists its programme is entirely peaceful.

Western strategy is increasingly focused on delaying Iranian progress, buying time in the hope that economic pressure will eventually lead to a change of mind in Tehran. The campaign has had some recent success in the form of covert sabotage – the Stuxnet computer worm, reportedly a US-Israeli project, slowed down Iran's uranium enrichment last year. Other efforts have involved finding bottlenecks in the Iranian programme and trying to tighten them.

Leaked US cables describe the outline of this containment policy. They reveal:

• Russia has vowed never to allow Iran to make its own fuel for a Russian-built nuclear power station at Bushehr.

• British officials have predicted Iran will amass 20 tonnes of low enriched uranium by 2014 – enough, if further enriched, to make 19 warheads.

• In 2008, Washington launched a worldwide campaign to cut off Iran's supplies of a high-strength steel, known as maraging steel, and carbon fibre. Access to these materials has been identified as a bottleneck in its enrichment programme.

Iran has always maintained its programme is intended for the generation of electricity, but its one nuclear power station, at Bushehr on the Gulf coast, is licensed to use only Russian-made fuel rods for its first 10 fuel loads, enough for just over 10 years.

At the April 2009 meeting in Vienna, Russian officials assured their colleagues that even after that period "Russia expects to supply fuel for the lifetime of Bushehr and it would not agree to provide Iran the necessary technology/licence to make its own fuel for the Bushehr reactor".

The experts at the Vienna meeting – from Britain, the US, Russia, France, China and Germany – noted that Iran had almost exhausted the original 531-tonne stock of the yellowcake it bought from South Africa in the 1980s.

According to the 2009 US embassy cable: "The French explained that Iran's current uranium stockpile was dwindling, with less than 100 tonnes of uranium remaining."

British experts argued however that the focus on uranium ore was a red herring as Iran had already converted enough of it into uranium hexafluoride (UF6) gas – the form in which uranium can be enriched in centrifuges – to keep the enrichment plant in Natanz running "for several years".

The 2009 Vienna meeting agreed Iran would need 1.5 tonnes of low-enriched UF6 to make an initial bomb, then another tonne for each subsequent device. The UK calculated that by 2014, Iran would have amassed 20 tonnes – enough for 19 warheads.

Britain officials now say that estimate might have to be lowered. Last year proved disastrous for Natanz – it had continual centrifuge breakdowns and suffered a complete shutdown for several days in November.

That was in part due to the Stuxnet computer worm but western officials say other sabotage operations have taken a toll, as has the inherent unreliability of the rudimentary and outdated IR-1 centrifuge design used by Iran.

The IR-1 is based on a design bought from Pakistan. It is built of a specialised aluminium and maraging steel, a rare commodity that has proved one of the most effective bottlenecks on the whole programme.

Olli Heinonen, formerly the IAEA's head inspector and now a senior fellow at Harvard University, said he believed Iran had enough material to make 12,000 centrifuges based on what is known about its overseas purchases. The estimate is shared by some European officials.

If correct, Iran has almost run out of centrifuge parts. It has made 11,000, of which 2,000 are believed to be broken and another 5,000 have been taken out of service. That leaves only 4,000 IR-1s enriching uranium at Natanz.

Efforts have now shifted to stopping Iran building new-generation centrifuges which use lighter carbon fibre rotors and are thought to be at least five times more efficient.

"This is really important for the future of the programme. IR-1s are relatively fragile and inefficient," Heinonen said.

Washington raised the alarm in 2008 and launched a worldwide campaign aimed at cutting off carbon fibre shipments to Iran. Cables were sent to US diplomats in European capitals and Beijing with lists of producers of carbon fibre and related equipment who were to be alerted to any suspicious orders.

A few months later, in the last days of the Bush administration, the state department was scrambling to head off a shipment to Iran of 25 tonnes of maraging steel. It is not clear whether it succeeded.

Ultimately, western officials admit Iran will find a way around any effort to stop the programme. At best, they say, it can only be delayed – its future will ultimately be decided by the leadership in Tehran.