WASHINGTON -- President Obama on Tuesday rejected an appeal by Prime Minister Benjamin Netanyahu of Israel to spell out a specific "red line" that Iran could not cross in its nuclear program, a senior administration official said, deepening the divide between the allies over how to deal with Iran's nuclear ambitions.
In an hourlong telephone conversation, this official said, Mr. Obama deflected Mr. Netanyahu's proposal to make the size of Iran's stockpile of close-to-bomb-grade uranium the threshold for a military strike by the United States against its nuclear facilities.
Mr. Obama, the official said, repeated the assurances he gave to Mr. Netanyahu in March that the United States would not allow Iran to manufacture a nuclear weapon. But the president was unwilling to agree on any specific action by Iran -- like reaching a defined threshold on nuclear material, or failing to adhere to a deadline on negotiations -- that would lead to American military action.
"We need some ability for the president to have decision-making room," said the official, who spoke on the condition of anonymity because of the delicacy of the talks. "We have a red line, which is a nuclear weapon. We're committed to that red line."
Israeli officials, however, say this guarantee may not be enough for Israel, which Iranian leaders have repeatedly threatened with annihilation. Diplomatic talks, the Israelis say, have done nothing to slow Iran's nuclear program -- nor have economic sanctions, though they have inflicted significant damage on the Iranian economy.
The telephone conversation came after a day that seemed to epitomize the frequently crossed wires between Mr. Obama and Mr. Netanyahu. It began with angry comments by the prime minister that the Obama administration had no "moral right" to restrain Israel from taking military action on its own if it refused to put limits on Iran. It continued with reports in the Israeli news media that the White House had rebuffed a request by Mr. Netanyahu's office for a meeting with Mr. Obama during the meeting of the United Nations General Assembly in New York this month. The White House denied those reports, citing more mundane scheduling problems. Finally, on Tuesday evening, Mr. Obama called Mr. Netanyahu.
The source of the conflict is the belief by Mr. Netanyahu that Iran, having continued to stockpile uranium enriched to 20 percent, is nearing the point at which Israel will no longer be able to prevent it from making a bomb.
Administration officials contend that the United States will still be able to detect, and prevent, Iran from passing that point. Nor does the administration have evidence that Iran's supreme leader, Ayatollah Ali Khamenei, has even made a decision to build a bomb. Iran, for its part, insists that its nuclear program is for peaceful energy purposes.
Israel's latest burst of anxiety about Iran comes in the midst of the American presidential election, leading some analysts to argue that Mr. Netanyahu is trying to use political leverage on Mr. Obama to stiffen his position. His Republican challenger, Mitt Romney, has accused Mr. Obama of not doing enough to protect a close ally.
Israeli officials flatly deny that Mr. Netanyahu is playing election-year politics. They said the prime minister was deeply frustrated by a recent interview with Secretary of State Hillary Rodham Clinton, in which she said the United States was "not setting deadlines."
People with close ties to Israel say Mr. Netanyahu and other Israeli officials are also frustrated because the Americans do not appear sufficiently concerned about Iran's growing stockpile of medium-enriched uranium. In its latest report, the International Atomic Energy Agency says the Iranians have amassed enough low- and medium-enriched uranium that, with further enrichment, could fuel as many as six nuclear weapons.
Basing a military judgment on Iran's stockpile of medium-enriched uranium could be tricky, however, because while the overall amount of this material has increased, the amount that can be readily used to fuel a bomb has declined since Iran converted some of it into plates to be used in a research reactor in Tehran.
"The Israelis are worried that once Iran accumulates a bomb's worth of 20 percent-enriched uranium, it's an easy dash to get weapons-grade nuclear fuel," said David Makovsky, a senior fellow at the Washington Institute for Near East Policy, who met recently with Israeli officials. "Before they decide they're on their own, I think they want to know that they and Washington see eye-to-eye that this is a red line that cannot be passed."
Indeed, Israeli officials appear reluctant to act without the backing of the United States. Mr. Netanyahu faces deep divisions within his own country about the wisdom of a military strike. On Thursday, Israel's deputy prime minister for intelligence and atomic affairs, Dan Meridor, appeared to undercut Mr. Netanyahu, saying in an interview with Israeli Army radio, "I don't want to set red lines or deadlines for myself."
David E. Sanger contributed reporting.
This article originally appeared in The New York Times.