By Fredrik Dahl
VIENNA (Reuters) - A U.N. nuclear agency investigation into suspected atomic bomb research by Iran will not be an "endless process", its chief said on Monday, pressuring Tehran to step up cooperation with the long-stalled probe.
Three weeks after Iran failed to meet a deadline for providing requested information to the International Atomic Energy Agency, Director General Yukiya Amano said Tehran needed to do much more to address the IAEA's concerns.
In line with the findings of a confidential IAEA report earlier this month, he said Iran had not carried out two of the five transparency steps it had agreed to implement by Aug. 25.
Lack of progress in the IAEA investigation could further complicate efforts by six world powers to negotiate a resolution to the wider, decade-old dispute with Iran over its nuclear work and persuade it to curb it in exchange for a gradual ending of sanctions.
"Iran needs to be as transparent as possible to clarify these issues," Amano told a news conference.
The U.N. agency would give an impartial and factual assessment to its 35-nation board on what the IAEA calls the possible military dimensions of Iran's nuclear program when it has a "good understanding of the whole picture", he said.
The investigation could be carried out within a "reasonable timeline" if Iran cooperated with the IAEA, he said, suggesting it could be done in roughly 15 months or less. "This is not an endless process," the veteran Japanese diplomat said.
Western diplomats say the IAEA at some stage would probably produce an assessment even if Iran - which denies it is seeking a nuclear weapons capability - were to stonewall the U.N. agency, although it would be based on incomplete information.
Iran has been promising to cooperate with the IAEA since Hassan Rouhani, seen as a pragmatist, was elected president last year. It says its nuclear program is for purely non-military purposes.
The two issues that have not yet been addressed by Iran under a cooperation pact with the IAEA are alleged experiments on explosives that could be used for an atomic device and also studies related to calculating nuclear explosive yields.
Amano earlier on Monday told the IAEA's governing board that Iran had "begun discussions" with the IAEA on these topics but he gave no details. He said the U.N. agency had also asked Iran to propose future transparency steps to help advance the investigation, but that it had yet to do so.
In 2011, the IAEA published a report that included intelligence indicating Iran had a nuclear weapons research program but halted it in 2003 when it came under increased international pressure. The intelligence suggested some activities may have resumed later. The report identified about 12 specific areas that it said needed clarification.
Iran says the allegations are baseless.
Last month, Amano met Rouhani and senior Iranian officials in Tehran in an attempt to make progress and the IAEA chief told Monday's board meeting that they had stated a "willingness to accelerate the resolution of all outstanding issues".
Rouhani's election raised hopes of a solution to the stand-off with the West after years of tension and fears of a new Middle East war. An interim accord was reached between Iran and the six powers in Geneva in November last year.
But Iran and the powers -- the United States, France, Germany, Britain, Russia and China -- did not meet a July target date for a comprehensive deal and now face a new deadline of Nov. 24, with talks due to resume in New York later this week.
Western officials say Iran must address the IAEA's concerns. They add that although there is no chance of the investigation being completed before the scheduled end of the six-power talks, some of the sanctions relief Iran is seeking would probably depend on its cooperation with the U.N. agency.
The IAEA is still not in a position to "conclude that all nuclear material in Iran is in peaceful activities", Amano said.
(Editing by Andrew Roche)