The Obama administration has assured Israel that Iran's process of converting nuclear material into a working weapon would take at least a year, The New York Times reported on Thursday.
The newspaper cited White House officials saying they believed the assessment had dimmed prospects of a preemptive Israeli strike against Iran's nuclear facilities within the next year.
Israel, widely assumed to have its own atomic arsenal, has hinted at military strikes, as a last resort, to deny Iran the means to make a nuclear bomb. Iran insists its nuclear program is peaceful.
The United States, the European Union and the UN Security Council have imposed sanctions on Iran over a nuclear program the West suspects is aimed at developing atomic weapons.
"We think that they have roughly a year dash time," Gary Samore, President Barack Obama's top adviser on nuclear issues, was quoted as saying, referring to the key question of how long it would take Iran to convert existing stocks of low-enriched uranium into weapons-grade material.
U.S. and Israeli officials now believe Iran would not be close to that stage anytime soon, the Times reported, citing administration officials who spoke on condition of anonymity.
Earlier this week, former U.S. envoy to the United Nations John Bolton told the Fox Business Network that Israel was unlikely to attack Iran's nuclear reactor in Bushehr, which will soon be online, adding that the Russian-built facility represents a major step forward for Iran's nuclear weapons aspirations.
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 the timeline is longer.
Last week, Russia said it will begin loading nuclear fuel into the reactor of Iran's first atomic power station on August 21, an irreversible step marking the start-up of the Bushehr plant after nearly 40 years of delays.
Russia agreed in 1995 to build the Bushehr plant on the site of a project begun in the 1970s by German company Siemens, but delays have haunted the e1 billion project and diplomats say Moscow has used it as a lever in relations with Tehran.
Despite U.S. criticism of the plan, Russia's nuclear chief said Thursday that the startup of Iran's first nuclear power plant will demonstrate that Iran is entitled to peaceful use of nuclear energy under international supervision.