Correction: An earlier version of this article misquoted Israeli Defense Minister Ehud Barak. The quote, which omitted a “not,” should have read: “The dividing line may pass not where the Iranians decide to break out of the nonproliferation treaty and move toward a nuclear device or weapon, but at the place ... that would make the physical strike impractical.” This version has been corrected.

Israeli Defense Minister Ehud Barak, right, walks with U.S. Defense Secretary Leon Panetta in Tel Aviv last October. Barak said that time is running out for stopping Iran’s nuclear advance, as the country’s uranium facilities disappear into newly constructed mountain bunkers. (Jack Guez/AFP/Getty Images)

Israeli leaders on Thursday delivered one of the bluntest warnings to date of possible airstrikes against Iranian nuclear sites, adding to the anxiety in Western capitals that a surprise attack by Israel could spark a broader military conflict in the Middle East.

Defense Minister Ehud Barak, speaking at a security forum attended by some of Israel’s top intelligence and military leaders, declared that time was running out for stopping Iran’s nuclear advance, as the country’s uranium facilities disappear into newly constructed mountain bunkers.

“Whoever says ‘later’ may find that later is too late,” Barak said. He switched from Hebrew to English for the last phrase: “later is too late.”

The language reflected a deepening rift between Israeli and U.S. officials over the urgency of stopping Iran’s nuclear program, which Western intelligence officials and nuclear experts say could soon put nuclear weapons within the reach of Iran’s rulers.

Although accepting the gravity of the Iranian threat, U.S. officials fear being blindsided by an Israeli strike that could have widespread economic and security implications and might only delay, not end, Iran’s nuclear pursuits.

In a series of private meetings with Israeli counterparts in recent weeks, Western officials have counseled patience, saying recent economic sanctions and a new European oil embargo are pummeling Iran’s economy and could soon force the country’s leaders to abandon the nuclear program. Yet Israelis are increasingly signaling that they may act unilaterally if there is no breakthrough in the coming months, according to current and former administration and intelligence officials.

“The Obama administration is concerned that Israel could attack Iranian nuclear facilities this year, having given Washington little or no warning,” said Cliff Kupchan, a former State Department official who specialized in Iran policy during the Clinton administration and recently returned from meetings with Israeli officials. He said Israel “has refused to assure Washington that prior notice would be provided.”

Defense Secretary Leon E. Panetta is one of several administration officials to express concern publicly that Israel is positioning itself for a surprise attack. Last month, the administration dispatched the Joint Chiefs chairman, Gen. Martin E. Dempsey, to the Israeli capital for high-level discussions about the possibility of a unilateral Israeli strike.

“Israel has indicated they’re considering this, and we have indicated our concerns,” Panetta told reporters Thursday after a NATO meeting in Brussels. Panetta declined to comment on published reports that he thinks the Israelis could carry out a strike this spring, possibly as early as April.

Although the Obama administration has not ruled out U.S. military action against Iran, White House officials are worried that a unilateral strike could shatter the broad international coalition assembled in the past three years to confront Iran over its nuclear program, which Iranian leaders have consistently said is for peaceful purposes.

U.S. officials fear that an attack by Israel could trigger Iranian retaliation not only against the Jewish state but also against American interests around the world. A prolonged conflict could disrupt oil shipments, drive up energy prices and devastate fragile Western economies, U.S. officials say.

Administration officials have hinted that the United States might not intervene militarily in a hostile exchange between Israel and Iran unless the conflict began to threaten U.S. forces or Israeli population centers. In an interview last month on CBS’s “60 Minutes,” Panetta said that in the event of an Israeli strike, U.S. military officials’ primary concern would be “to protect our forces.”

British Deputy Prime Minister Nick Clegg also expressed concern Thursday that Israel was moving closer to a decision on a potentially destabilizing military strike.

“Of course I worry that there will be a military conflict and that certain countries might seek to take matters into their own hands,” Clegg told the House magazine, a weekly British political journal.

Clegg, whose government recently imposed new sanctions against Iran’s central bank, said Britain was convinced that “ there are very tough things we can do which are not military steps in order to place pressure on Iran.”

At Thursday’s Israeli security conference, in the resort city of Herzliya, Barak and other Israeli officials pointed to recent moves by Iran to begin enriching uranium at a second plant, located in a bunker built into a mountain near the city of Qom. Once that facility is complete, deterring Iran will be far more difficult, they say.

“The dividing line may pass not where the Iranians decide to break out of the nonproliferation treaty and move toward a nuclear device or weapon, but at the place . . . that would make the physical strike impractical,” Barak said.

He rejected criticism that Israeli leaders had failed to consider the full implications of military action. “There is no basis for the claim that this subject. . . was not discussed with appropriate breadth and depth,” he said.

“The assessment of many experts around the world, not only here, is that the result of avoiding action will certainly be a nuclear Iran, and dealing with a nuclear Iran will be more complicated, more dangerous and more costly in lives and money than stopping it,” he said.

Speaking at the same conference, the chief of military intelligence, Gen. Aviv Kochavi, said Iran already has enough fissile material to build four nuclear weapons and could do so within a year if Iranian leaders give the order. U.S. intelligence officials have concluded that Iran’s supreme leader, Ayatollah Ali Khamenei, has adopted a course of gradually gathering the components necessary for nuclear weapons while deferring a decision on whether to build and test a bomb.

Although there have been no indications in Israel that a military strike is imminent, Israeli officials have conveyed a sense of urgency, suggesting that a window of opportunity for a military strike is closing.

Barak, in a meeting with German Foreign Minister Guido Westerwelle, urged that diplomatic efforts to halt the Iranian nuclear program “be conducted intensively and urgently” and that tougher sanctions target Iran’s financial system and central bank, as well as its oil exports.

Israeli officials warn that beyond posing an existential threat to Israel, Iran’s possession of a nuclear weapon could trigger a regional nuclear arms race in the volatile Middle East and alter Israel’s strategic position in the region.

Warrick reported from Washington. Staff writers Craig Whitlock and Karen DeYoung contributed to this report.