Persuading Iran to scale back its enrichment of uranium, which can be used for peaceful purposes but also for nuclear weapons, will be the key aim of world powers in upcoming talks, analysts say.

The US, China, Russia, Britain, France and Germany may also press Tehran, at the meeting in Istanbul on Saturday, to give the UN nuclear watchdog more access to its facilities and answer accusations of a covert weapons programme.

Uranium enrichment is key because it is the "most pressing priority in terms of Iran's ability to make a weapon quickly if it decided to do so," Peter Crail, analyst at the Arms Control Association think-tank in Washington, told AFP.

At present Iran has some 3,000 kilos (6,600 pounds) of low-enriched uranium (LEU), of 3.5-percent purity, but of greater concern is the country's rapidly expanding capacity to enrich to 20 percent at its virtually impregnable Fordo site.

Western powers fear that if Iran were to take the decision to develop the bomb, it could relatively quickly reconfigure Fordo's centrifuges to enrich to weapons-grade levels of 90 percent.

The talks between Iran and the six world powers this weekend will be the first such discussions since a similar meeting broke up inconclusively more than a year ago.

The immediate aim of the upcoming talks is to "build confidence" that Iran will not be able to make this "quick sprint", said Mark Fitzpatrick from the International Institute for Strategic Studies in London.

Iran says that 20-percent uranium is to make fuel plates for the Tehran Research Reactor (TRR) in order to produce medical isotopes used to diagnose cancers and other diseases.

One option at the upcoming talks would be a revival of previous attempts at a deal whereby other countries supply fuel for the TRR in return for Iran suspending 20-percent enrichment and sending some of its stockpiles abroad.

But for Mark Hibbs at the Carnegie Endowment for International Peace, this would be a "non-starter" for Iran without a "roadmap" for sanctions to be loosened and Tehran being allowed to retain its peaceful nuclear activities.

Fitzpatrick believes that another possible confidence-building measure might be "technical measures" that make it more difficult for Iran to convert its centrifuges at Fordo or at Natanz, another site, to 90-percent production.

"The IAEA should also be allowed to detect enrichment levels at Fordo and Natanz on a real-time basis, without only having to rely on sending environmental samples back to Austria for time-consuming analysis," he said.

Another concession could be for Iran to implement the so-called "additional protocol" of the nuclear non-proliferation treaty that Iran, as a NPT signatory, briefly adhered to but dropped in 2006, analysts said.

This would give the IAEA the right to access sites that at present are off-limits and help soothe one of the biggest fears about Iran's nuclear programme, namely that it has secret sites -- as Fordo was until 2009.

The IAEA itself regularly says in its quarterly reports that since Iran is "not providing the necessary cooperation" it is "unable to provide credible assurances about the absence of undeclared nuclear material and activities."

Last but not least, analysts said, the P5+1 powers may press that Iran has to provide answers to a major IAEA report in November that heightened suspicions over Tehran's activities.

That report cited "overall, credible" evidence from different sources that at least until the end of 2003, and possibly since, Tehran carried out "activities ... relevant to the development of a nuclear explosive device."

So far, Iran has dismissed out of hand -- most recently in two high-level IAEA visits in late January and February -- all the allegations contained in the report, saying the findings were based on evidence fabricated by its enemies.

Most notably, Iran has denied the IAEA access to the Parchin military base near Tehran where the agency's report said it may have tested explosives for warhead research in a large metal container.

Hibbs also said that the West should steer clear of raising other issues in the talks such as human rights.

"Iran may conclude that non-nuclear issues are intended by the West to be a tool to destabilise and destroy the Islamic republic, and then not negotiate," the analyst told AFP.