The Obama administration, citing evidence of continued troubles inside Iran's nuclear program, has persuaded Israel that it would take roughly a year - and perhaps longer - for Iran to complete what one senior official called a "dash" for a nuclear weapon, according to U.S. officials.
Administration officials said they believe the assessment has dimmed the prospect that Israel would pre-emptively strike against the country's nuclear facilities within the next year, as Israeli officials have suggested in thinly veiled threats.
For years, Israeli and U.S. officials have debated whether Iran is on an inexorable drive toward a nuclear bomb and, if so, how long it would take to produce one. A critical question has been the time it would take Tehran to convert existing stocks of low-enriched uranium into weapons-grade material, a process commonly known as "breakout."
Israeli intelligence officials had argued that Iran could complete such a race for the bomb in months, while U.S. intelligence agencies have come to believe in the past year that the timeline is longer.
"We think that they have roughly a year dash time," said Gary Samore, President Barack Obama's top adviser on nuclear issues, referring to how long it would take the Iranians to convert nuclear material into a working weapon. "A year is a very long period of time."
Mr. Samore said the United States believed international inspectors would detect an Iranian move toward breakout within weeks, leaving a considerable amount of time for the United States and Israel to consider military strikes.
The U.S. assessments are based on intelligence collected over the past year, as well as reports from international inspectors.
Now, U.S. and Israeli officials believe breakout is unlikely anytime soon. For one thing, Iran, which claims it is interested in enriching uranium only for peaceful purposes, would be forced to build nuclear bombs from a limited supply of nuclear material, currently enough for two weapons. Second, such a decision would require kicking out international weapons inspectors, eliminating any ambiguity about Iran's nuclear plans.
Even if Iran were to choose this path, U.S. officials said it would probably take Iran some time to reconfigure its nuclear facilities to produce weapons-grade uranium and ramp up work on designing a nuclear warhead.
Israeli officials have indicated that if they saw a race for the bomb under way, they would probably take military action and encourage the United States to join the effort. A spokesman for the Israeli Embassy in Washington declined to comment for this article. In interviews, Israeli officials said their assessments were coming into line with the American view, but they remain suspicious that Iran has a secret enrichment site yet to be discovered.