Pentagon: Iran needs 3-5 years to build usable atom bomb

A technician at the uranium conversion facility in Isfahan, south of Tehran, February 3, 2007.

WASHINGTON (Reuters) - Iran could produce enough highly enriched uranium for a single nuclear bomb in as little as one year but would probably need three to five years to assemble, test and deploy a "usable" atomic weapon, top Pentagon officials said on Wednesday.

The new timeframe presented to Congress comes as President Barack Obama presses a reluctant China to back swift sanctions on Iran and U.S. intelligence agencies sought to complete a new National Intelligence Estimate that will assess Tehran's nuclear progress.

Lieutenant General Ronald Burgess, director of the Defense Intelligence Agency, said available information suggested 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.

The United States believes Iran has yet to make the decision to shift production to highly enriched uranium, according to officials.

But when asked how long it would take Iran to produce enough highly enriched uranium for 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 U.S. 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 the one-year estimate referred only to how long it would take Iran to produce enough fissile material for a weapon. Far more time would be needed to finish work on a bomb, such as assembling and testing, officials said.

"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 told the panel.

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.

MILITARY OPTION OUT OF FAVOR

Michele Flournoy, under secretary of defense for policy, said President Barack Obama has made clear that "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 said.

"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."

Cartwright said taking military action against Iran might delay its nuclear development but was unlikely to be decisive in stopping the program for good.

While voicing confidence that the U.S. military could undertake a large-scale operation that involves physically occupying Iran to ensure the nuclear program is disbanded, he cautioned: "I think that there would be consequences to our readiness and to the challenges that we already face in this nation economically to pay for a war."

The United States is now fighting wars in Iraq and Afghanistan, both bordering Iran.

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 (NIE) 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 timeframe.

Burgess said production of a new NIE was currently under way but that "the decision on when it will be released and when it will be finished has not been determined yet."

Jane Harman, chairwoman of the U.S. House of Representatives Homeland Security Subcommittee on Intelligence, said on Tuesday that the new NIE was "essentially complete".

(Editing by Cynthia Osterman)