VIENNA (Reuters) — Any chance of defusing a standoff over Iran’s nuclear programme may hinge on the definition of a key precondition given to Tehran — suspension of nuclear “enrichment-related activity”.Six world powers have offered a package of incentives to Iran not to purify nuclear fuel and are pushing for an early response from Tehran, which the United States and European Union allies fear is trying to develop an atom bomb.

A breakthrough looks doubtful now. Iran says it is enriching uranium only for power generation and rules out scrapping the programme even if there are talks. The powers rule out talks unless Tehran freezes the programme first to establish trust.

But behind public fronts, there have been discussions aided by International Atomic Energy Agency (IAEA) experts in Vienna on whether there can be flexibility in defining “suspension” in order to start talks, a diplomat close to the debate said.

“The whole issue, the only issue, is to find a definition of suspension that allows both sides to say they got their way. Otherwise, this package goes nowhere,” the Vienna diplomat said.

He said one scenario weighed was to let Iran, under heightened IAEA surveillance, run a small number of centrifuge enrichment machines empty, without the uranium “UF6” gas normally injected into them for enrichment into fuel.

This could be termed a “maintenance pause”, or “standby”, as mooted by some US nuclear analysts, he said. It might enable “pre-negotiations” on terms for implementing the incentives and avoid the word “suspension”, which to Iran implies externally-imposed preconditions it calls humiliating.

“After recent talks between IAEA and US officials, the IAEA was informally asked to evaluate the relative proliferation risks of Iran being allowed to run their (current) cascade of 164 centrifuges, and a couple more they are building,” he said.

“The analyses sent back to Washington concluded there would be little to no risk since any quantities of enriched uranium yielded would be so small as to not be relevant to bombmaking.”

But one analysis read out to Reuters by a Western diplomat concluded that even spinning centrifuges empty would give Iran know-how for “a successful, long-term centrifuge operation”.

US officials insist the definition of suspension is fixed — all activities fostering fuel work — and are not bargaining over this with Iran. They balk at being drawn into talks giving Iran time to expand enrichment and make it a fait accompli.

“Questions asked of the IAEA were by no means meant to open some channel to Iran or show any intention to change our position. The IAEA’s own analyses undercut their political argument (for compromise),” said the Western diplomat.

But one US official who tends to take a hard line on Iran said there were perceptions the Bush administration would eventually give in on the issue. There is a precedent. Western leaders originally ruled out any enrichment ever on Iranian soil as too dangerous. Now, to ensure Russia and China closed ranks with the four Western powers on the new approach to Iran, the package offered gives Tehran leeway to enrich in the future, after a confidence-building moratorium.

Of the other six powers, only Britain appears dead set against softening the definition of enrichment activity. IAEA diplomats say compromise is needed because Iran has already mastered basic enrichment technology while the West’s mooted option of sanctions if Tehran rejects the offer risks a Russian or Chinese veto in the UN Security Council.

A compromise might assuage Iranian national pride, sharpened by past imperial power domination of its oil and gas wealth, and allay world fears of nuclear proliferation in Iran.

Tehran has hinted it could cap the number of centrifuges spinning short of “industrial capacity”, or thousands needed to enrich uranium at sufficient volume to produce bombs, and reinstate short-notice IAEA inspections.

Nuclear analysts said Iran was likely now to ask the powers to define an enrichment halt as “spinning without feeding”.

“This could confront the West with a tough decision. China and Russia will want to say this is not enrichment so it’s a basis for a deal,” said Mark Fitzpatrick of London’s International Institute for Strategic Studies.

“But as long as centrifuges spin, Iran is gaining crucial expertise that would enable them to prevent accidents, leaks or other telltale signs of secret activity,” he said.