The UN atomic agency, after a year of false starts, finally expects to sign a long-elusive deal with Iran on January 16, but that will be the easy part, experts say.

Implementing the accord will be a lengthy and fragile process that will only partially resolve a decade-long standoff over Iran's nuclear programme and help to silence Israeli "drums of war".

Implementing the accord between the IAEA and Iran "will take years", one Vienna diplomat said, adding: "It's not going to be solved overnight."

The International Atomic Energy Agency's announcement on Friday after "good" talks in Tehran that it expects a deal in January was something of a surprise after a string of previous fruitless meetings, and considerable scepticism remains.

"We have been down this road before," analyst and former US State Department official Mark Fitzpatrick, now at the International Institute for Strategic Studies, told AFP, citing previous optimistic IAEA predictions in January and May that came to nothing.

The IAEA says it wants Iran to allow it to visit sites, talk to officials and examine documents related to activities "related to the development of a nuclear explosive device" -- mostly in the past but possibly still ongoing.

The IAEA, in a November 2011 report, cited "overall, credible" evidence indicating that such activities took place until 2003, and possibly since.

They included experiments with high explosives, tests with surrogate nuclear materials to similate a nuclear explosion, modelling and engineering studies on ballistic missile payloads and a "green salt project" to enrich uranium covertly.

But questions persist over the sources of the information.

The IAEA said its sources included a "foreign expert" -- thought to be a former Soviet nuclear scientist who worked in Iran -- in addition to satellite images, open-source documents and the nuclear trafficking network of Pakistan's Abdul Qadeer Khan -- as well as information from Iran itself.

But the bulk came from foreign intelligence services, including some 1,000 pages of documents known as the "alleged studies" from a so-called "laptop of death" reportedly obtained by the CIA in 2004 that several observers have thrown into doubt.

Iran has denounced the IAEA's alleged evidence as either forgeries provided by its "enemies" or as non-nuclear -- therefore none of the IAEA's business. It also complains that the IAEA refuses even to show it some of the material.

But the IAEA maintains that Iran has a case to answer, and in part to get around the foreign intelligence issue it has sought access to the Parchin military base because its information on the base is "independent".

"Believe me, we are reasonable people," IAEA head Yukiya Amano said this month. "Bypassing these issues with a military dimension or refusing to respond to questions is not the solution."

-- Structured approach --

Iran wants to agree what both sides call a "structured approach" document -- IAEA jargon for a work plan to deal with the accusations in a methodical fashion.

This has however proved to be a sticking point because Iran wants to include elements that some of the IAEA's member states fear might "sacrifice the IAEAs authority to get information it needs", said Mark Hibbs from the Carnegie Endowment for International Peace.

One Vienna diplomat for example said Friday after being briefed by the IAEA after its talks in Tehran the day before that Iran wants the IAEA to agree to "close" an issue for good once it has been addressed.

Another thorny issue would be how to respond if in the course of implementing the IAEA-Iran arrangement it turns out that some of the IAEA's "evidence" of past weapons research work is indeed accurate.

Such an explosive revelation could blow diplomatic efforts to resolve the crisis out of the water -- quite literally.

One way around this is what Pierre Goldschmidt, a former IAEA deputy director general, has called a "grace period" allowing Iran to admit past transgressions in return for a promise that it will not be punished.

"Such disclosures could be very beneficial for confidence-building," Goldschmidt said in May. "It would help persuade the international community that this time, Tehran has indeed opted for full cooperation and transparency."

The IAEA is not empowered to grant such a "grace period"; instead this would have to be one element of a wider deal between Iran and the big powers, who are thought to be pushing for a new meeting soon.