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VIENNA (Reuters) - Either Iran could build a nuclear bomb in a matter of months or it is unlikely to get such a weapon any time soon -- depending on which Western expert you talk to.
The differing estimates show the difficulty in trying to assess how long it could take Iran to convert its growing uranium stockpile into weapons-grade material and how advanced it may be in other areas vital for any bomb bid.
The answers to those questions could determine the major powers' room for manoeuvre in trying to find a diplomatic solution to a dispute over Iran's nuclear ambitions which has the potential to spark a wider conflict in the Middle East.
Iran says its nuclear programme is for peaceful purposes. Western-based analysts generally agree with their governments that Tehran is developing technology that could be used to make a bomb, but they disagree about just how close it is to success.
U.S. defence analyst Greg Jones gave one of the more urgent warnings this month, arguing that if Iran decides to make a bomb it could produce enough highly-enriched uranium (HEU) in about eight weeks.
"The timeframe will shrink to only about four weeks by the end of next year as Iran's enriched uranium stockpiles and enrichment capacity continue to increase," Jones, of the conservative Nonproliferation Policy Education Centre, said.
Iran "needs to be treated as a de facto nuclear power simply by virtue of being so close to having a weapon," he added in an article in U.S. political magazine New Republic.
Other experts say such estimates are unrealistic, given the hurdles Iran must still overcome.
"I think that we tend to overstate sometimes how close Iran is to being able to develop a nuclear weapon," said senior researcher Shannon Kile at the Stockholm International Peace Research Institute, a think-tank based in the Swedish capital.
"I just don't see how you can credibly say they are going to be eight weeks away or even 18 months away."
Jones is not the only expert to suggest that Iran may be very close to producing the refined uranium material necessary for a weapon, should it decide to do so.
A paper published by the U.S. Bipartisan Policy Centre think-tank said Iran could make 20 kg of HEU -- a quantity it said would be enough for one device -- in two months.
It said it remained unclear if Iran had mastered the technology to turn the HEU into a weapon, but that history suggested this could be achieved in less than six months.
But another Washington-based think-tank, the Institute for Science and International Security (ISIS), said Jones's calculation method was "unreliable" and a breakout in such a short time at Iran's Natanz enrichment site was not realistic.
Other experts stressed that Iran would also need to turn any weapons-usable uranium into the core of a nuclear missile if it wanted more than a crude device, adding to the timetable.
Mark Fitzpatrick, of the International Institute for Strategic Studies (IISS), said he now believed Iran could make a nuclear weapon in less than two years' time.
"Suggestions that Iran will be able to produce weapons in a matter of months are irresponsible," Fitzpatrick, a director of the IISS Non-proliferation and Disarmament Programme, said.
But, "just as exaggeration is irresponsible, so too is complacency," he added.
Iran's refusal to halt its enrichment activities has drawn four rounds of U.N. sanctions since 2006.
Refined uranium can be used to fuel nuclear power plants or provide material for bombs if processed much further.
The West fears that Iran's move last year to enrich uranium to a fissile purity of almost 20 percent -- up from the 3.5 percent normally needed for reactors -- takes it significantly closer to the 90 percent level needed for arms.
Iran says it needs this higher-grade material for a reactor producing radioactive isotopes to treat cancer patients.
ISIS said that in the fastest scenario, Iran could have enough of the 20 percent material for a nuclear weapon in 2012 if it refined more.
But even if Iran were to produce bomb-grade uranium, it would also have to transform it from gaseous into metal form, miniaturise it to squeeze into the nose cone of a missile and fit it with a trigger system.
Sanctions and possible sabotage -- such as the Stuxnet computer virus and killings of nuclear scientists that Tehran blames on Israel -- may have slowed Iran's atomic work, but its stockpile of uranium is steadily growing.
Iran "is moving ahead in all of the ways that you would need to if you wanted a nuclear weapon," Fitzpatrick said.
Raising the pressure, U.N. nuclear watchdog chief Yukiya Amano this month said he was "increasingly concerned" about possible work in Iran to develop a nuclear missile. He hoped to give more details soon about the basis for those concerns.
Israel and the United States, Tehran's arch foes, have not ruled out military action if diplomacy fails to resolve the row. Israel bombed an Iraqi reactor in 1981 and launched a similar sortie against Syria in 2007.
"Israel has no doubt that Iran is pursuing nuclear weapons," the head of the Jewish state's atomic energy commission, Shaul Chorev, told member states of the U.N. nuclear agency last week.
Israel's chief of military intelligence, Aviv Kochavi, said in January that Iran could produce bombs within two years.
Iran and Arab states say Israel itself has an atomic arsenal that threatens regional peace and stability. Israel neither confirms nor denies that it possesses nuclear arms.
Diplomatic efforts to seek a negotiated outcome with Iran have been deadlocked since a fruitless meeting in January.
Tehran now says it is prepared to resume the talks. Western countries are sceptical, but the six powers involved -- the United States, China, Russia, France, Britain and Germany -- may once again test its readiness to engage on issues of substance.
They have offered economic and political incentives for Iran to drop enrichment, so far in vain. Iran's says it is its "inalienable right" to develop the nuclear fuel cycle.
Greg Thielmann, a senior fellow at the U.S.-based Arms Control Association, stressed the importance of using the time available to influence decision-making in Tehran: "A nuclear-armed Iran is neither imminent nor inevitable."