An important recent development in Iran's nuclear programme, if it continues, might help to ease international fears that Tehran wants the bomb, but serious questions still remain, analysts and diplomats said.

This potentially positive step, as highlighted in recent quarterly reports by the International Atomic Energy Agency, concerns uranium enriched by Iran to a fissile purity of 20 percent.

This material is of major international concern because if further purified to 90 percent -- a process well within Iran's technical capabilities -- it would be suitable for a bomb.

According to the IAEA's most recent report last week, Iran has produced 324 kilogrammes (714 pounds) of 20-percent enriched uranium, well above the around 240 kilogrammes thought to be needed for one nuclear device -- which is reportedly also Israel's "red line".

But more than 40 percent of this has been converted into another form, triuranium octoxide, which experts say is tricky -- although not impossible -- to convert back to the original uranium hexafluoride.

Iran says that it is converting this uranium in order to provide fuel for a reactor in Tehran, and four others that outgoing President Mahmoud Ahmedinejad last February ordered constructed, for nuclear medicines.

Tehran also calls it a "confidence-building" measure in so-far fruitless talks with six world powers on hold until after Iranian presidential elections on June 14.

But the problem, says Mark Hibbs, analyst at the Carnegie Endowment for International Peace, is that Iran's output is "way beyond" what it needs. Plans for four more reactors are also "pie in the sky," he told AFP.

At the same time, the rate at which Iran is converting the 20-percent enriched uranium remains below the production rate -- meaning that the size of the overall stockpile continues to creep ever higher.

Moreover, Iran's output of 20-percent enriched uranium is set to triple once new machinery at its Fordo enrichment facility is up and running.

Iran is also putting in more modern enrichment machines at its Natanz plant, used to enrich uranium to five-percent purities for nuclear power, which will enable Tehran to process fissile material more quickly.

As the IAEA cannot vouch for Iran's activities being peaceful, its 35-nation board of governors, which meets in Vienna from Monday, has passed several resolutions calling on Tehran to suspend all enrichment, as has the UN Security Council.

Iran refuses, calling the UN resolutions -- and related UN and Western sanctions, which last year began to cause it economic problems -- illegal and the IAEA, which derives some 65 percent of its budget from Washington and its allies, "politicised".

-- Plutonium --

But even if Iran manages to soothe some of the concerns about uranium, there are also other areas of worry.

Not least of these is progress, as outlined in the last IAEA report, in building the new IR-40 reactor at Arak, which Western countries fear could provide Iran with plutonium, an alternative to uranium for a weapon, if the fuel is reprocessed.

"Everyone is always focusing on the uranium enrichment, and for understandable reasons, but there is this second pathway," said Shannon Kile, nuclear expert at the Stockholm International Peace Research Institute (SIPRI).

"The IR-40 is of the general type that was used by the North Koreans, the Indians and the Pakistanis in their initial programmes, so it is well suited to producing plutonium for weapons purposes," Kile told AFP.

Arak "shows that this issue is not just about 20-percent enriched uranium stockpiles. This is a broader picture," agreed one senior Western diplomat in Vienna.

Another bone of contention meanwhile is what the IAEA suspects may have been Iranian research, mostly before 2003 but possibly ongoing, into creating a nuclear payload for a missile.

Iran denies this, and 10 meetings with the IAEA since its major November 2011 report summarising these claims -- based mostly, but not only, on foreign intelligence -- have failed to make progress.