WASHINGTON U.S. intelligence agencies believe Iranian leaders have not yet decided to build a nuclear bomb, and some officials say recent problems affecting Tehran's nuclear equipment and personnel have set back Iran's nuclear program by two years or more.
The latest assessments, based at least in part on Israeli intelligence, appear to have eased political pressures on Israeli and American leaders for a military strike against Iran's nuclear infrastructure, according to current and former officials familiar with the intelligence.
These developments have also given the administration of President Barack Obama breathing room to pursue a two-pronged strategy of seeking greater diplomatic engagement with Tehran while also threatening increased economic sanctions, they said.
Deadly attacks on Iranian nuclear scientists and a computer virus which allegedly infected control systems for Iran's uranium enrichment equipment have likely slowed Iran's nuclear progress, Israeli intelligence sources have said. That evaluation is shared by some, but not all, U.S. nuclear and intelligence experts.
"We've got more time than we thought," said Gen. Michael Hayden, former director of the Central Intelligence Agency.
Hayden said he now believes the "key decision point" for possible military action against Iran has been postponed until the "next (U.S.) presidential term" which would be after the 2012 election.
At the same time, current and former U.S. national security and intelligence officials believe Iran is actively trying to assemble the infrastructure and know-how for atomic bomb production if and when political leaders decide to build one.
A current U.S. official who is following the issue closely told Reuters: "The intelligence folks think that the Iranians aren't necessarily moving full steam ahead with the development of a nuclear weapon, but that there's fairly robust debate inside the Iranian regime on whether to go forward."
"This is a momentous decision for an isolated government, and people are watching very closely to see what they do."
The official added that, "Even if (the Iranians) choose to do the wrong thing and proceed toward nuclear weapons, it's unclear that they could do so quickly. While they've got a lot of knowledge, putting it into practice is a whole different ball game."
Six major powers -- the United States, Britain, Russia, Germany, France, and China -- are meeting with Iran next week in Istanbul to seek assurances that it is not trying to develop nuclear weapons. Tehran says its nuclear work is for production of electricity.
"BREAKOUT CAPABILITY" SOUGHT
For years, a key point of debate among analysts has been estimating how quickly Iranian scientists and engineers could build a bomb once political leaders gave the word.
The most recent -- and controversial -- consensus of U.S. spy agencies issued in 2007 reported that Iran had "halted its nuclear weapons program" in the autumn of 2003, although Tehran was "keeping open the option to develop nuclear weapons."
The 2007 National Intelligence Estimate suggested it was conceivable Iran might be able to produce enough bomb-grade uranium to build a weapon at the earliest in 2010. But U.S. agencies believed the Iranians' ability to achieve this was more likely after 2015.
Some Israeli intelligence experts strongly disagreed, suggesting publicly following the report's release that Iran might be able to build a bomb within months rather than years.
The 2007 assessment took the steam out of efforts by some hard-liners in the administration of President George W. Bush to seek harsher sanctions, or even military action against Iran.
Despite criticism from conservatives, U.S. intelligence agencies stuck by the 2007 assessment until the revelation in 2009 of Iran's secret underground enrichment facility near the holy city of Qom.
That discovery and other classified intelligence alarmed the intelligence community and forced a re-evaluation of when Iran could reach a "breakout capability" threshold, making it capable of assembling a nuclear weapon.
A consensus began to develop among U.S. government experts that Iran may well have resumed the kind of nuclear related research and development activities that could hasten the timeline toward building a bomb.
MAJOR ISRAELI SHIFT
In the last few days, however, the assessment in the United States and Israel seems to be shifting back toward the 2007 intelligence evaluation of slower Iranian nuclear progress.
Israelis, who had claimed Iran's bomb-making was advanced enough to produce a device within a matter of months, appeared to significantly revise their outlook.
Meir Dagan, outgoing director of Israel's principal intelligence agency, Mossad, said Tehran would not be able to build a bomb for at least four years "because of measures that have been deployed against them."
Israel sees a nuclear-armed Iran as a threat to its existence. It bombed an Iraqi reactor in 1981 and a suspected Syrian nuclear site in 2007 to disrupt nuclear programs in those two Arab states.
Israel is widely assumed to have the Middle East's only atomic arsenal, but many analysts say its air force is too small to take on Iran's nuclear sites on its own.
Following the Israeli statements, word began to circulate among U.S. intelligence officials about a new push to complete the long-awaited updated National Intelligence Estimate on the Iranian nuclear program.
An official with the Office of the Director of National Intelligence would not comment, citing a long-standing policy not to discuss these reports or even acknowledge their existence.
Some American experts question whether the revised Israeli view of Iran's nuclear glitches could be too optimistic and Israeli Prime Minister Benjamin Netanyahu also seemed to question the Israeli intelligence view.
A few days after Dagan's assessment, Netanyahu insisted the Iranians were still intent on getting a nuclear weapon and that only a combination of sanctions and a credible threat of military action would be effective deterrents.
David Albright, a former United Nations weapons inspector who heads the Institute for Science and International Security, a Washington think tank, told Reuters that his own analysis still indicated Iran's nuclear research could reach a breakout point for bomb building in a year or two.
Albright said he did not understand why Israelis like Dagan were so confident Iran will remain incapable of putting together a bomb any earlier than 2015.
(Editing by Jackie Frank)