WASHINGTON — When President Obama and Prime Minister Benjamin Netanyahu of Israel met in the White House on Monday, the main subject was how to calibrate the resumption of negotiations with Iran while continuing to accelerate sanctions and sabotage against its nuclear program. But they remained divided on two central questions: If Iran decided to race for a nuclear weapon, would the West detect that in time to stop it? And even if it were detected, would an airstrike be the best option?

From the administration’s top ranks down, American officials say they would almost certainly detect a sprint to manufacture a weapon. Among the signs, they say, would be any move to evict international inspectors, satellite evidence of any testing of the kinds of conventional explosives used to develop a bomb and, quite likely, reports from the West’s contacts inside the Iranian scientific community of a change in the actions of the laboratories run by Mohsen Fakhrizadeh, the Iranian believed to be leading the weaponization program.

The Israelis say a bolt for the bomb might be detectable but might not be. And they say that everything from the history of the Holocaust to the tight geography of the Middle East demands that they assess the risk of missing the turn in Iran much more critically than Washington does.

“Amazingly, some people refuse to acknowledge that Iran’s goal is to develop nuclear weapons,” Mr. Netanyahu said Monday evening in a speech to the American Israel Public Affairs Committee, a hawkish, pro-Israel lobbying group, dismissing the American arguments that Iran may be seeking only the option to build the ultimate weapon. “You see, Iran claims that it’s enriching uranium to develop medical research. Yeah, right.”

American intelligence officials, by contrast, say Iran has not made a final decision on whether to build a bomb.

In meetings with Mr. Obama’s top national security advisers in recent weeks, Mr. Netanyahu has been cagey about defining when Iran would — by Israel’s definition of the problem — get so close to a nuclear capability that he would judge a military strike the only available option. Instead, he has been warning about the dangers of getting involved in negotiations with Iran that he believes it could use to deflect pressure and to make it far harder for Israel to threaten an attack. The United States and five other nations announced Tuesday that they had agreed to reopen talks with Tehran, though expectations for progress are low.

Mr. Obama has said from his days as a candidate for president that he would enter such talks without conditions. Mr. Netanyahu views that as foolish to the extreme. Speaking in Canada before he arrived in Washington, Mr. Netanyahu demanded that before any negotiations begin, Iran must take steps that would preclude it from progressing toward a bomb. Mr. Obama’s advisers say the chances of Iran’s agreeing to that at the outset of a negotiation are zero. And they insist that there is time to try another round of negotiations.

“I have zero doubt that if Iran attempted a breakout, we’d see it,” one senior administration official said several weeks ago. But in the past, there have been doubts. Two years ago, Robert M. Gates, then the secretary of defense, asked: “If their policy is to go to the threshold but not assemble a nuclear weapon, how do you tell that they have not assembled? I don’t actually know how you would verify that.”

The truth is that the answer to the question is unknowable. While American intelligence agencies famously misjudged that Saddam Hussein was advancing on a bomb project when he had none, they also have a long record of missing signs that countries were getting very close to a bomb. They missed the timing of the first Soviet nuclear test in 1949, to President Harry S. Truman’s outrage. They also got the timing wrong on China in the 1960s, India in the ’70s and Pakistan in the ’80s. To this day, even after North Korea has conducted two nuclear tests, no one is sure whether the country’s engineers actually know how to make and deliver a real, working bomb.

The Israelis cite this sorry record to suggest that the Americans are overstating their capabilities. “The Israeli view is that because they have less capability to deal with Iran, they have less time to allow this to go on,” one senior American official said. “They think that because we have more capability, we have more time.”

That argument forces the debate between Mr. Obama’s camp and Mr. Netanyahu’s into a second question: Is an air attack the best way to set the Iranian program back?

The Obama administration asks what would be accomplished if an attack set Iran back by only three or four years, unified even the country’s opposition leaders against the West and pushed the program further underground. As Rolf Mowatt-Larssen, a former official of the Central Intelligence Agency and the Energy Department with a deep background in nuclear proliferation, put it on Tuesday: “Both sides say we don’t want Iran to get the bomb. Where there’s a difference of opinion is that we don’t think an airstrike solves the problem, and the Israelis do.”