A mooted deal between the IAEA and Iran got a cool reception Tuesday on the eve of talks with world powers aimed at defusing the dangerously escalating crisis over Tehran's nuclear programme.

Yukiya Amano, International Atomic Energy Agency head, said on returning from Tehran that he and Iran's chief nuclear negotiator made a "decision" to reach an agreement on the UN watchdog probing suspected weapons activities.

But contrary to the hopes of some diplomats before he left on Sunday, Amano failed to actually sign a deal, saying at Vienna airport that this would happen "quite soon" because of remaining, unspecified "differences."

The United States expressed caution, saying the proposed agreement marked a "step forward" but that Tehran would be judged on its actions.

"It's important to note that the announcement today is a step forward," White House spokesman Jay Carney said, calling the planned deal between the IAEA and Iran "certainly significant."

However, "we will make judgments about Iran's behavior based on actions."

A key demand of world powers is that Iran address accusations in a major IAEA report in November that, until 2003, and possibly since, Tehran did work "relevant to the development of a nuclear explosive device."

One Western diplomat told AFP there had been "no breakthrough" in Amano's visit. Another said the trip appeared disappointing but that they were waiting for a "clearer picture" at meetings in Vienna later on Tuesday.

"This is only a promise, and Iran has made many, many promises in the past," said a third, adding that Tehran was possibly trying to appear cooperative ahead of Wednesday's meeting in Baghdad.

The US mission in Vienna said that while it appreciated Amano's efforts, it was "concerned by the urgent obligation for Iran to take concrete steps to cooperate fully" with the agency, a point underlined by the State Department.

Washington will look for Tehran to "provide the access to all of the locations, the documents, and the personnel that the IAEA requires in order to determine whether Iran's program is exclusively for peaceful purposes," State Department spokeswoman Victoria Nuland told reporters.

Israel is "highly sceptical" about the deal, a senior official told AFP on Tuesday, with Finance Minister Yuval Steinitz saying Iran has played "hide-and-seek for years" with the international community and the IAEA.

Mark Hibbs, proliferation expert at the Carnegie Foundation for International Peace, told AFP "the negotiation isn't over and done with until it's signed on the dotted line."

"Amano has to be extremely careful he doesn't forfeit any rights to Iran for the sake of getting an agreement. That would serve as a bad precedent."

Amano also said access to the Parchin military site near Tehran, where the IAEA believes some weapons work took place, and which the agency has made a point of seeking to inspect, would "be addressed."

On Wednesday, the P5+1 -- Britain, China, France, Russia and the US plus Germany -- hope Iran will agree to a series of steps that can allay once and for all suspicions that it wants the bomb, most notably uranium enrichment.

Tehran says its nuclear programme is for purely peaceful purposes.

In particular, they would like a suspension of enrichment to 20 percent, a capability that in theory makes it relatively easy to enrich to weapons grade -- 90 percent -- if it decided to develop a nuclear arsenal.

Iran on Tuesday announced it was loading domestically produced, 20-percent enriched uranium fuel into its Tehran reactor, underlining its atomic progress.

Shipping its uranium stockpiles abroad, and implementing the additional protocol of the nuclear Non-Proliferation Treaty, which allows for more intrusive IAEA inspections, are considered other ways in which Iran could build confidence.

But Iran will likely be disappointed if it expects to see sanctions relief in return for any of these moves, with the most it can hope for being a pledge -- with strings attached -- not to impose any more, diplomats said.

In any case, it is far from certain that any firm promises will be made from either side in Baghdad, with one envoy playing down expectations by saying that even if the talks go well, the results might not be "tangible."

Instead, the outcome could be an agreement to hold more regular talks at a working level to thrash out the technical details of confidence-building measures, a process needing two vital and elusive elements: patience and trust.

Israel, meanwhile, widely believed to have nuclear weapons itself, is concerned the P5+1 might eventually tolerate Iran enriching uranium, something which it has made clear is a "red line."

"Iran threatens Israel, peace and the entire world. Against this malicious intention, the world's leading countries must show determination, not weakness," Premier Benjamin Netanyahu said late on Monday.

In Baghdad, meanwhile, a sandstorm closed the airport, casting doubt on whether the talks would begin on schedule and be completed in one day.

Iran's nuclear programme: some thorny issues
Vienna (AFP) May 22, 2012 - Iran was due to meet Wednesday in Baghdad with the P5+1 powers -- the United States, China, Russia, Britain, France and Germany -- to seek a way out of their long-running dispute over Tehran's nuclear programme. Herewith a short summary of the main issues:

- IRAN AND THE BOMB: Iran says its nuclear programme, which has expanded dramatically in recent years, is for peaceful purposes, to generate electricity and provide medical isotopes. Supreme leader Ayatollah Ali Khamenei said in February that possession of a nuclear bomb "constitutes a major sin," reiterating a religious edict he made in 2005. Many in the international community suspect the programme however to be a front for a covert nuclear weapons drive, with the UN Security Council passing four rounds of sanctions on the Islamic republic.

- RIGHTS AND DUTIES: As a 1968 signatory of the nuclear non-proliferation treaty, Iran says it has the right to a peaceful nuclear programme. However the treaty also stipulates Iran has to submit to inspections by the UN nuclear watchdog, the International Atomic Energy Agency, to determine that no materials or sites are being used for other purposes. The IAEA says that a lack of Iranian cooperation means it is unable to guarantee that this is the case.

- URANIUM ENRICHMENT: The trickiest part of making a nuclear weapon is creating the fissile core, either uranium or plutonium. Iran has for some time been enriching uranium to 3-5 percent purities needed for power generation, but in 2010, it began processing to 20 percent, and since January it had done so at the Fordo site in a mountain near the holy city of Qom. This takes it significantly closer to weapons-grade of 90 percent and reduces the "breakout" time needed to make a bomb -- if it decided to do so.

- MILITARY DIMENSIONS: A major IAEA report in November 2011 substantially increased suspicions that at least until 2003, and possibly since, Iran has done research into how to actually make a nuclear bomb. The report, which cited intelligence from several countries as well as the IAEA's own data and Iranian information, has been rejected by Iran as based on forgeries. The IAEA also says Iran refused it access in two visits earlier this year to the Parchin military site where some of this work is alleged to have been carried out.

- FUEL SWAP: One way of cooling tensions would be to revive earlier ideas whereby Iran would ship out its stockpiles of enriched uranium to a foreign country, thereby easing fears that it could be further enriched and used in a nuclear weapon, and be supplied in return with fuel pads for the Tehran Research Reactor, which makes medical isotopes.

- ADDITIONAL PROTOCOL: Another way of building confidence in Tehran's nuclear programme would be for Iran to implement the additional protocol of the NPT, something it briefly adhered to but then rejected in 2006. This would give the IAEA greater inspection rights and help soothe one of the biggest concerns, namely that the country has sites the IAEA does not know about.