Timeline on Iran bomb narrows, but barely

U.S. Secretary of Defense Leon Panetta walks across the tarmac towards his C-17 plane during his departure from Baghdad December 15, 2011.

WASHINGTON (Reuters) - How long would Iran need to develop a nuclear weapon if its leaders decided to build one?

Defense Secretary Leon Panetta thinks it could take just under a year, an ever so slightly more narrow timeline than the one-year minimum his predecessor, Robert Gates, put forward.

But nuclear experts are skeptical Tehran could move that quickly and a Pentagon spokesman acknowledged on Tuesday he was not aware of any new intelligence driving Panetta's latest remarks, which aired on U.S. television on Monday evening.

Asked whether Iran could have a nuclear weapon in 2012, Panetta told CBS News: "It would probably be about a year before they can do it. Perhaps a little less."

Again, that is if Iran decides to build a bomb - a decision U.S. officials do not believe its leadership has taken. Iran says its nuclear program is entirely for peaceful purposes.

Panetta warned that if Iran had secret enrichment sites U.S. intelligence was unaware of, the timeline could shrink.

Hans Kristensen, director of the nuclear information project at the Federation of American Scientists, described Panetta's projection as unlikely.

"It's too short a timeline," Kristensen said, adding Iran would need more time to enrich its uranium to weapons grade and then build a working nuclear device.

"I think we're talking about a few years away if they decided" to build a weapon, he added, cautioning that he did not have access to any classified intelligence.

Panetta's remarks come at a moment of intense speculation about whether Israel, which sees a nuclear-armed Iran as an existential threat, may be moving closer to military action to halt a program it says is aimed at building an atomic bomb.

Captain John Kirby, a Pentagon spokesman, noted Panetta told CBS the nuclear weaponization process "often takes longer than people usually anticipate."

Pentagon officials, including Panetta's predecessor, have usually described the window as between one and three years.

"This idea that they could be, once they make a decision, a year or more away is not a new idea," Kirby said, even as he acknowledged Panetta's remarks indicated Tehran could move forward in potentially less than a year.

The United States is advocating sanctions and diplomatic pressure to isolate Iran over its nuclear program.

Panetta has repeatedly warned about unintended consequences of a military strike on Iran, including fallout to the global economy and likely retaliation against U.S. forces in the region.

He has also warned it would only delay, not derail, the nuclear program and could trigger a backlash in Iran that could bolster its leaders.

Kirby stressed that Panetta's views on the dangers of a strike were unchanged.

Still, in the interview, Panetta renewed the U.S. position that a nuclear-armed Iran was unacceptable. He used strong language, saying it was a "red line for us and that's a red line, obviously, for the Israelis."

"If we have to do it, we will deal with it," he said. Asked whether he meant military steps, Panetta replied: "There are no options off the table."

(Reporting By Phil Stewart; Editing by John O'Callaghan)