* U.N. nuclear agency and Iran to meet on Jan. 16
* Both sides reported progress in last month's talks
* But IAEA chief says "outlook is not bright"
By Aaron Sheldrick
TOKYO, Jan 11 (Reuters) - The U.N. nuclear agency chief said on Friday he was not optimistic about talks with Iran next week on getting access to a military base Western powers suspect has been used for atom bomb-related work.
The comments by Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), contrasted with a more upbeat assessment given by the Vienna-based U.N. agency after a meeting with Iranian officials last month.
The IAEA, whose mission it is to prevent the spread of nuclear weapons in the world, has been trying for a year to negotiate a so-called structured approach with Iran that would give it access to sites, officials and documents.
The IAEA's priority is to visit the Parchin military facility southeast of Tehran, where the agency suspects explosives tests relevant for nuclear weapons may have taken place, perhaps a decade ago. Tehran denies this.
"The outlook is not bright," Amano said in Tokyo, referring to the negotiations to be held in Tehran on Wednesday on the framework accord the IAEA hopes will enable it to quickly resume its stalled investigation into suspected atom bomb research.
The talks between the IAEA and Iran are separate from, but linked to, broader diplomacy by six world powers to resolve the nuclear row with Iran before it leads to a Middle East war, feared because of Israeli threats to bomb Iranian nuclear sites.
Western powers say Iran is trying to develop the capability to make atomic weapons, a charge the Islamic Republic rejects.
Both the IAEA and Tehran have said progress was achieved at the December meeting, without giving details.
However, Amano said in Japanese comments translated into English: "Talks with Iran don't proceed in a linear way. It's one step forward, two or three steps back ... So we can't say we have an optimistic outlook" for the Jan. 16 meeting.
WINDOW OF OPPORTUNITY?
Western diplomats say Iran has worked for the past year to cleanse Parchin of any evidence of illicit activities, but Amano said late last year an IAEA visit would still be useful.
The IAEA said after last month's talks in Tehran it expected a deal could be completed in January and swiftly implemented.
But Western diplomats in Vienna later said stumbling blocks remained, including Iran's demand for access to intelligence documents that form part of the basis for the IAEA's concerns.
Even if a deal is reached, the diplomats said, it remained to be seen how it would be carried out. Western officials have often accused Iran of stonewalling IAEA investigations.
"An agreement is a good first step, but implementation is the most important part," one Western envoy said on Friday.
Iran's refusal to curb nuclear activity with dual civilian and military applications, and its lack of openness with the IAEA, have drawn tough Western punitive measures and a threat of pre-emptive military strikes by its arch-adversary Israel.
Analysts and diplomats say there is a window of opportunity for world powers to make a renewed diplomatic push to find a broader negotiated solution to the dispute after U.S. President Barack Obama won re-election in November.
The six powers - the United States, Britain, France, Germany, Russia and China - want Iran to scale back its uranium enrichment programme and cooperate fully with the IAEA. Iran wants the West to lift sanctions hurting its economy.
Both sides say they want to resume talks this month, but have yet to agree on a date and venue.