U.S. and Israel Split Over How to Deter Iran WASHINGTON - Amid mounting tensions over whether Israel will carry out a military strike against Iran's nuclear program, the United States and Israel remain at odds over a fundamental question: whether Iran's crucial nuclear facilities are about to become impregnable. Israel's defense minister, Ehud Barak, coined the phrase "zone of immunity" to define the circumstances under which Israel would judge it could no longer hold off from an attack because Iran's effort to produce a bomb would be invulnerable to any strike. But judging when that moment will arrive has set off an intense debate with the Obama administration, whose officials counter that there are other ways to make Iran vulnerable. Senior Israeli officials, including the foreign minister and leader of the Mossad, have traveled to Washington in recent weeks to make the case that this point is fast approaching. American officials have made reciprocal visits to Jerusalem, arguing that Israel and the West have more time and should allow sanctions and covert actions to deter Iran's plans. The Americans have also used the discussions to test their belief, based on a series of public statements by Israeli officials, that an Israeli strike against Iran could come as early as spring, according to an official familiar with the discussions. President Obama tried to defuse arguments for military action in a telephone call last month with Prime Minister Benjamin Netanyahu of Israel, the substance of which was confirmed by an Obama administration official who spoke only on the condition of anonymity because he was not authorized to describe the conversation. While the two men have had an often contentious relationship over Middle East diplomacy, American officials emerged from that exchange persuaded that Mr. Netanyahu was willing to give economic sanctions and other steps time to work. The difference of opinion over Iran's nuclear "immunity" is critical because it plays into not just the timing - or bluffing - about a possible military strike, but the calculations about how deeply and quickly sanctions against Iran must bite. If the Israeli argument is right, the question of how fast the Iranians can assemble a weapon becomes less important than whether there is any way to stop them. " "Zone of immunity" is an ill-defined term," said a senior Obama administration official, expressing frustration that the Israelis are looking at the problem too narrowly, given the many kinds of pressure being placed on Tehran and the increasing evidence that far tougher sanctions are having an effect. The Israelis have zeroed in on Iran's plan to put much of its uranium enrichment near Qum in an underground facility beneath so many layers of granite that even the Pentagon acknowledges it would be out of the reach of its best bunker-busting bombs. Once enrichment activities are under way at Qum, the Israelis argue, Iran could throw out United Nations inspectors and produce bomb-grade fuel without fear the facility would be destroyed. At its core, the official said, the argument the Israelis make is that once the Iranians get an "impregnable breakout capability" - that is, a place that is protected from a military strike - "it makes no difference whether it will take Iran six months or a year or five years" to fabricate a nuclear weapon, he said. The Americans have a very different view, according to a second senior official who has discussed the concept with Israelis. He said "there are many other options" to slow Iran's march to a completed weapon, like shutting off Iran's oil revenues, taking out facilities that supply centrifuge parts or singling out installations where the Iranians would turn the fuel into a weapon. Administration officials cite this more complex picture in pressing the Israelis to give the latest sanctions a chance to inflict enough pain on the Iranian leadership to force it back to the negotiating table, or to make the decision that the nuclear program is not worth the cost. Iran's currency has plunged, they note; its oil is piling up in storage tanks because it cannot find buyers, and there is growing evidence of fissures among the country's leadership.