Although repeated rounds of sanctions over many years have not dissuaded Iran from pursuing nuclear technology, an administration official involved in the Iran policy said the hope was that the current troubles “give us a window to impose the first sanctions that may make the Iranians think the nuclear program isn’t worth the price tag.”

While outsiders have a limited view of Iran’s nuclear program, the Obama administration officials said they believed that the bomb-development effort was seriously derailed by the exposure three months ago of the country’s secret enrichment plant under construction near the holy city of Qum. Exposure of the site deprived Iran of its best chance of covertly producing the highly enriched uranium needed to make fuel for nuclear weapons.

In addition, international nuclear inspectors report that at Iran’s plant in Natanz, where thousands of centrifuges spin to enrich uranium for nuclear fuel, the number of the machines that are currently operating has dropped by 20 percent since the summer, a decline nuclear experts attribute to technical problems. Others, including some European officials, believe the problems may have been accentuated by a series of covert efforts by the West to undermine Iran’s program, including sabotage on its imported equipment and infrastructure.

These factors have led the administration’s policy makers to lengthen their estimate of how long it would take Iran to accomplish what nuclear experts call “covert breakout”  the ability to secretly produce a workable weapon.

“For now, the Iranians don’t have a credible breakout option, and we don’t think they will have one for at least 18 months, maybe two or three years,” said one senior administration official at the center of the White House Iran strategy. The administration has told allies that the longer time frame would allow the sanctions to have an effect before Iran could develop its nuclear ability.