JERUSALEM - Israel believes Iran will not be able to produce a nuclear bomb before 2015 and a top Israeli official has counselled against pre-emptive military strikes, intelligence assessments published on Friday showed.

Given in a briefing by Mossad director Meir Dagan upon his retirement on Thursday, the assessments pointed to new Israeli confidence in U.S.-led sanctions and covert action designed to discourage or delay Tehran's uranium enrichment programme.

They were also in line with Israeli Prime Minister Benjamin Netanyahu's circumspection -- echoing misgivings voiced more publicly by the Obama administration -- about resorting to force against Iran, which denies seeking nuclear arms and has vowed to retaliate against Israel and U.S. interests for any such attack.

"Iran will not achieve a nuclear bomb before 2015, if that," Dagan said, according to a transcript obtained by Reuters.

Dagan, who in June 2009 told Israeli lawmakers that Iran could have its first nuclear warhead by 2014, attributed his valedictory timeline to a variety of factors including domestic ferment in Iran and the bite of international sanctions.

Iran's enrichment drive has also suffered technical setbacks, possibly a sign of foreign sabotage in incidents such as the apparent corruption of some Iranian nuclear machinery by the Stuxnet computer worm.

Western intelligence agencies similarly say Iran could make a bomb by the middle of the decade, should it choose to enrich uranium to higher levels and master weaponisation techniques.

Iran says it is refining uranium only to the lower level of fissile purity suitable for electricity and medical isotopes.

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Dagan, a former general whose eight-year tenure as spymaster was widely seen as having escalated Mossad shadow wars, said any Israeli military action against Iran should be last-ditch only.

Such attacks could spur Iran to pull out of the nuclear Non-Proliferation Treaty and pursue its programme entirely free of U.N. inspections, he said. Iran's past secrecy over some of its projects stirred suspicions of undeclared military designs.

"Israel should not hasten to attack Iran, doing so only when the sword is upon its neck," Dagan said. His briefing was reported on prominently by Israeli newspapers and broadcasters.

Gerald Steinberg, chief scholar at the Programme of Conflict Management and Negotiation at Israel's Bar-Ilan University, noted that Israel, like the United States, had not formally ruled out the military option for dealing with Iran.

"But the immediacy, the need, the concern that 2009 or 2010 would be the last opportunity has clearly been shown to not be the case, and my guess is from listening to the statements that have been made, that up to 2015 there is not likely to be a need for using military force," he told Reuters Television.

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 distant, dispersed and fortified sites alone.

Israel is also wary of drawing reprisals from Iran or its allies in Syria and among Lebanese and Palestinian guerrilla groups. Washington has said it does not want a new regional war.

Israeli Defence Minister Ehud Barak said on Tuesday that "the time is still one of diplomacy" for resolving the Iran crisis.

"What is required is much harsher sanctions than today's," he said in a speech. "Only such paralysing sanctions have a chance of putting the Iranian will to the test."