By Maayan Lubell and Matt Spetalnick
JERUSALEM/WASHINGTON (Reuters) - Prime Minister Benjamin Netanyahu said Israel and the United States were in talks on setting a "clear red line" for Iran's nuclear program, but the two allies remained at odds on Monday over whether to spell out a clear threshold for military action against Tehran.
The Israeli leader, who has been pressing President Barack Obama for a tougher line against Iran, again signaled that a sharper U.S. ultimatum for Tehran could deter it from developing nuclear weapons and mitigate the need for a military response.
Netanyahu's recent calls for world powers to set clear markers that would show they were determined to stop Tehran's nuclear drive has suggested a growing impatience with the United States, Israel's main ally.
Washington, which has resisted the idea of laying down red lines for Iran in the past, has urged the Israeli leader to give diplomacy and sanctions imposed on the Islamic Republic more time to work to rein in Iran's nuclear work peacefully. But Obama has not ruled out military action if all else fails.
Recent heightened Israeli rhetoric has stoked speculation that Israel might attack Iran before the U.S. elections in November, believing that Obama would give it military help and not risk alienating pro-Israeli voters.
In his latest call for an unambiguous message on boundaries that Tehran must not cross, Netanyahu said in interview with Canada's CBC television aired late on Sunday: "We're discussing it right now with the United States." But he has yet to define publicly what he wants.
Senior U.S. officials offered no sign that the United States and Israel were any closer to narrowing their differences.
Secretary of State Hillary Clinton made comments that were interpreted by Israeli media as rejecting Netanyahu's call for a red line and drew admonishment from a senior Israeli official.
In an interview with Bloomberg Radio, Clinton was asked about Netanyahu's demand that a red line be set and also whether she thought a time limit should be put on negotiations with Iran over its nuclear program.
Clinton responded only to the second part of the question, saying, "We're not setting deadlines." Israeli media seized on those comments as a rejection of Netanyahu's red-line demand.
The Israeli official said: "These statements will not stop Iran's centrifuges from spinning."
Obama, who has had a strained relationship with Netanyahu, is facing accusation from Republican presidential nominee Mitt Romney that he is lax in his support for Israel and not hard enough on Iran.
WHITE HOUSE AVOIDS 'SPECIFICITY'
White House spokesman Jay Carney dodged questions on whether Obama was ready to offer new red-line assurances to Israel.
"The line is the president is committed to prevent Iran from acquiring a nuclear weapon, and he will use every tool in the arsenal of American power to achieve that goal," he told reporters, reiterating Obama's position.
Asked whether that meant the United States would act only if Iran began building a bomb, Carney said: "It is not fruitful as part of this process to engage in that kind of specificity."
Netanyahu has faced opposition at home to any go-it-alone attack. Opinion polls show a majority of Israelis do not want their military to strike Iran without U.S. support.
"I don't think that they (Iran) see a clear red line, and I think the sooner we establish one, the greater the chances that there won't be a need for other types of action," Netanyahu told CBC, apparently referring to military steps.
"If Iran saw that, there's a chance, I won't say it's guaranteed, but there's a chance they might pause before they cross that line."
Israel and the West believe Iran is working toward nuclear weapon development capability. Israel, widely thought to be the Middle East's only atomic power, says a nuclear-armed Iran would be a threat to its existence. The Islamic Republic says its nuclear work is for peaceful energy purposes only.
The Jewish state says little time remains before Iran achieves a "zone of immunity" in which Israeli bombs would be unable to penetrate deeply buried uranium enrichment facilities.
The United States has more potent weapons that would allow more time for the sanctions push to work.
Israeli newspaper Haaretz said on Monday that Netanyahu had told German Foreign Minister Guido Westerwelle that if Iran enriched uranium above 20 percent, that would provide a red line, proving Tehran had chosen to exceed the level of refinement suitable for civilian energy and "break out" with an atom bomb.
Enrichment to 90 percent fissile purity is the typical threshold for weapons-grade nuclear fuel. Haaretz said Netanyahu stressed that from the moment Iran decided to make a nuclear bomb, it would need only six weeks to enrich to 90 percent.
Many independent analysts say, however, that Iran would need additional time - from several months to a year or more - to fashion weapons-grade material into a nuclear warhead and fit it onto a missile capable of delivering the payload.
Netanyahu is scheduled to travel to the New York and address the U.N. General assembly about Iran later this month.
A meeting with Obama, who is deep in his re-election campaign and due to speak to the forum two days before Netanyahu arrives, has not been finalized, the Israeli official said.
(Editing by Jeffrey Heller, Michael Roddy and Mohammad Zargham)