Iran could produce enough highly enriched uranium needed for a single nuclear bomb in as little as one year but would probably need three to five years to build a "usable" atomic weapon, top Pentagon officials said on Wednesday.
The new timeframe presented to Congress comes as US President Barack Obama presses a reluctant China to back swift sanctions on Iran and US intelligence agencies try to finish a classified report assessing how Tehran's nuclear program is progressing.
Lieutenant General Ronald Burgess, director of the Defense Intelligence Agency, said information that has been made public suggests that the centrifuges at Iran's enrichment plant at Natanz were producing low-enriched uranium and were not yet being used to make highly enriched uranium at a level needed for nuclear weapons.
Asked how long it would take Iran to produce enough highly enriched uranium to produce a single nuclear weapon if the leadership decided to do so, Burgess told the Senate Armed Services Committee: "The general consensus - not knowing again the exact number of centrifuges that we actually have visibility into - is we're talking one year."
General James Cartwright, vice chairman of the US military's Joint Chiefs of Staff, concurred: "They have enough low enriched uranium now that, if they further processed and enriched that, that in a year ... they would have enough material for one weapon."
But Cartwright cited "several caveats," saying the one-year estimate referred to how long it could take Iran to produce enough fissile material for a weapon and does not include the time needed for "assembling it, the testing, all the things that go into it."
"Experience says it is going to take you three to five years" to move from having enough highly enriched uranium to having a "deliverable weapon that is usable,... something that can actually create a detonation, an explosion that would be considered a nuclear weapon," Cartwright said.
Western powers fear Iran wants to develop nuclear weapons under the cover of a civilian atomic program. Tehran says its program is intended only for peaceful power generation.
Michele Flournoy, Under Secretary of Defense for Policy, told the same Senate committee that US President Barack Obama has made clear "all options are on the table" to rein in Iran's nuclear program.
"We see it as the Department of Defense's responsibility to plan for all contingencies and provide the president with a wide range of military options should they become necessary," she told the Senate panel
"But ... military options are not preferable and we continue to believe that the most effective approach at this point in time is the combination of diplomacy and pressure," Flournoy said.
Views within the intelligence community have long varied on how long it will take Iran to build a nuclear weapon.
A 2007 National Intelligence Estimate judged with "moderate confidence" that Iran would "probably" be technically capable of producing enough highly enriched uranium for a weapon sometime in the 2010-2015 time frame.
Jane Harman, chairwoman of the US House of Representatives Homeland Security Subcommittee on Intelligence, said on Tuesday that a revised US National Intelligence Estimate (NIE) on Iran was "essentially complete".