* Rough estimate agreed among Western spy agencies
* Diplomats: 18-month estimate is "worst-case scenario"
UNITED NATIONS, Oct 26 (Reuters) - Intelligence agencies estimate that it would probably take Iran a minimum of 18 months to develop a nuclear weapon if it chose to build one, Western diplomats and intelligence officials said.
For years the U.S. Central Intelligence Agency, Britain's MI6, Israel's Mossad, their French and German counterparts and other spy agencies have been struggling to penetrate Iran's secretive nuclear program, often disagreeing internally and with each other on when Iran could have a nuclear weapon.
Tehran insists that its nuclear program is peaceful and says Western spies are lying when they suggest Iran is pursuing nuclear weapons. Some officials at the U.N. nuclear watchdog in Vienna have warned against exaggerating the case against Iran, as happened with prewar Iraq.
But several Western diplomats told Reuters that the top spy agencies generally agreed that Tehran would need at least 18 months to build an atomic weapon if it decided to make one -- a much shorter timeline than some of the agencies' publicly released assessments of Iran's nuclear plans.
"It's not a formal assessment or formal agreement but a rough agreement that we can all work with more or less," one Western diplomat told Reuters on condition of anonymity. He said it was a "worst-case scenario," not the most likely one.
Another Western diplomat confirmed the agreement, adding that the assessment was based on the assumption that Tehran would need at least six months to purify its uranium stocks to weapons-grade level and another 12 months for "weaponization" -- building the actual nuclear weapon.
The minimum possible timeline is crucial because it gives an indication of how much time the six countries spearheading efforts to persuade Iran to halt its enrichment program -- the United States, Britain, France, Germany, Russia and China -- have before Tehran could theoretically have an atomic weapon.
Iran has so far rejected offers of economic and political incentives from the six in exchange for suspending enrichment, despite getting hit with three rounds of U.N. sanctions.
A plan under discussion in Vienna that would move most of Iran's low enriched uranium stocks to Russia and France for enrichment and fabrication into fuel rods would add another 12 months onto the timeline if Tehran accepts it, the diplomats said. Tehran needs a specialized fuel assembly for a medical reactor but is reluctant to send its uranium abroad.
The diplomats also pointed out that the 18-month estimate did not account for technical obstacles and bottlenecks that could be expected to slow down the process of building an actual weapon. Nor does it assume Tehran has already made a strategic decision to build such a weapon.
U.S. Director of National Intelligence said in February that Iran would not realistically be able to a get a nuclear weapon until 2013. Mossad Chief Meir Dagan was more cautious, saying recently that it would take the Iranians until 2014.
But an Israeli official linked to the country's security cabinet described the 18-month timeline as "reasonable."
A recently retired Israeli government intelligence analyst who still has access to briefings also said the reasoning was solid: "You can argue about the timeline -- a few months here or there -- but that's not relevant to the big picture."
David Albright, a former U.N. weapons inspector and head of the Institute for Science and International Security think-tank, said it was in line with information he has.
"It's consistent with what I was told by a senior European intelligence official," he said.
But one Western intelligence official expressed doubt that Iran would be able to produce a bomb so quickly, describing the 18-month minimum timeline as unrealistic. "Do they have the knowledge and wherewithal to produce highly enriched uranium now?" the official said. "I'm skeptical."
The diplomats who described the timeline said there was much about Iran's nuclear program that intelligence agencies and the Vienna-based U.N. International Atomic Energy Agency (IAEA) were ignorant about because of Tehran's secretiveness.
"We are all very mindful of what happened in Iraq," one diplomat said. "There is so much we don't know."
One of the justifications for the U.S.-led invasion of Iraq in March 2003 had been U.S. and British assertions -- now known to have been erroneous -- that Iraq's late leader Saddam Hussein had revived his clandestine nuclear arms program.
The diplomats said it was very possible Iran had another undeclared enrichment plant hidden somewhere, similar to the recently exposed site near Qom which IAEA inspectors visited for the first time on Sunday. The Qom site's existence was revealed last month by the United States, Britain and France.
One intelligence official told Reuters that even if Iran had another such plant it would probably not be able to produce significant quantities of enriched uranium and would therefore not have much of an impact on the presumed timeline.
The diplomats said Western intelligence agencies continued to have their disagreements on Iran, above all regarding the U.S. 2007 National Intelligence Assessment (NIE) that concluded Iran had ended its nuclear weaponization program in 2003.
Israeli and European intelligence experts disagree with that assessment and believe Iran's research on fabricating a nuclear weapon has continued. The diplomats said U.S. intelligence agencies were considering revising the 2007 NIE. (Additional reporting by Adam Entous in Washington, Dan Williams in Jerusalem and Mark Heinrich in Vienna; Editing by Eric Walsh)
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