TEL AVIV — With diplomatic efforts to curb Iran’s nuclear program teetering, Israel’s defense minister has ordered his forces to prepare a military option, warning the world that Israel would take matters into its own hands if a new nuclear agreement did not sufficiently constrain Iran.

But several current and former senior Israeli military officials and experts say that Israel lacks the ability to pull off an assault that could destroy, or even significantly delay, Iran’s nuclear program, at least not anytime soon. One current high-ranking security official said it would take at least two years to prepare an attack that could cause significant damage to Iran’s nuclear project.

A smaller-scale strike, damaging parts of the program without ending it entirely, would be feasible sooner, experts and officials say. But a wider effort to destroy the dozens of nuclear sites in distant parts of Iran — the kind of attack Israeli officials have threatened — would be beyond the current resources of the Israeli armed forces.

“It’s very difficult — I would say even impossible — to launch a campaign that would take care of all these sites,” said Relik Shafir, a retired Israeli Air Force general who was a pilot in a 1981 strike on an Iraqi nuclear facility.

“In the world we live in, the only air force that can maintain a campaign is the U.S. Air Force,” he said.

The recent discussion of a military attack on Iran is part of an Israeli pressure campaign to make sure that the countries negotiating with Iran in Vienna do not agree to what Israeli officials consider “a bad deal,” one that in their view would not prevent Iran from developing nuclear weapons.

At the moment, there appears to be little chance of that as the talks, aimed at resurrecting the 2015 nuclear agreement with Iran, have only regressed since Iran’s new hard-line government rejoined them last month.

Until now, Israel has tried to curb Iran’s nuclear program, which it considers an existential threat, through a combination of aggressive diplomacy and clandestine attacks. Israeli officials considered it a coup when they were able to persuade President Donald J. Trump to withdraw from the 2015 agreement, which President Biden now wants to salvage.

Israel has also waged a shadow war through espionage, targeted assassinations, sabotage and cyberattacks — smaller-scale operations that it has never formally claimed. Israel secretly considered mounting full-scale airstrikes in 2012 before abandoning the plan.

But as Iran’s nuclear enrichment program approaches weapons-grade levels, Israeli politicians have warned in increasingly open fashion what the world has long assumed: that Israel could turn to open warfare if Iran was allowed to make progress toward developing a nuclear weapon, a goal Iran denies.

In September, the head of the Israeli armed forces, Lt. Gen. Aviv Kochavi, said large parts of a military budget increase had been allocated to preparing a strike on Iran. Early this month, the Mossad chief, David Barnea, said Israel would do “whatever it takes” to stop Iran from making a nuclear bomb.

This month, during a visit to the United States, Defense Minister Benny Gantz publicly announced that he had ordered the Israeli Army to prepare for a possible military strike on Iran.

But Israeli experts and military officials say that Israel currently lacks the ability to deal Iran’s nuclear program a knockout blow by air.

Iran has dozens of nuclear sites, some deep underground that would be hard for Israeli bombs to quickly penetrate and destroy, Mr. Shafir said. The Israeli Air Force does not have warplanes large enough to carry the latest bunker-busting bombs, so the more protected sites would have to be struck repeatedly with less effective missiles, a process that might take days or even weeks, Mr. Shafir added.

One current senior security official said Israel did not currently have the ability to inflict any significant damage to the underground facilities at Natanz and Fordow.

Such an effort would be complicated by a shortage of refueling planes. The ability to refuel is crucial for a bomber that may have to travel more than 2,000 miles round trip, crossing over Arab countries that would not want to be a refueling stop for an Israeli strike.

Israel has ordered eight new KC-46 tankers from Boeing at a cost of $2.4 billion but the aircraft are back-ordered and Israel is unlikely to receive even one before late 2024.

Aside from the ability to hit the targets, Israel would have to simultaneously fend off Iranian fighter jets and air-defense systems.

Any attack on Iran would also likely set off retaliatory attacks from Hezbollah in Lebanon and Hamas in Gaza, allies of Iran that would try to force Israel to fight a war on several fronts simultaneously.

Iran’s defense capabilities are also much stronger than in 2012, when Israel last seriously considered attacking. Its nuclear sites are better fortified, and it has more surface-to-surface missiles that can be launched swiftly from tunnels.

“It is very possible that when the Israeli planes try to land back in Israel, they will find that the Iranian missiles destroyed their runways,” said Tal Inbar, an aviation expert and former head of the Fisher Institute for Air and Space Strategic Studies, an aviation-focused research group.

Other military experts, however, say that Israel could still take out the most important elements of the Iranian nuclear apparatus, even without newer aircraft and equipment.

“It’s always good to replace a car from 1960 with a brand-new car from 2022,” said Amos Yadlin, a former air force general who also participated in the 1981 strike. “But we have refueling capabilities. We have bunker busters. We have one of the best air forces in the world. We have very good intelligence on Iran. We can do it.

“Can the American Air Force can do it better? Definitely. They have a much more capable air force. But they don’t have the will.”

He cautioned that he would only support a strike as a last resort.

Israeli officials refuse to discuss the red lines Iran must cross to warrant a military strike. However, a senior defense official said that if Iran were to begin enriching uranium to 90 percent purity, weapons-grade fuel, Israel would be obliged to intensify its actions. American officials have said Iran is currently enriching uranium up to 60 percent purity.

The fact that it could take years to ramp up a program to carry out a massive air campaign against Iran should come as no surprise to Israeli military officials. When Israel considered such an attack in 2012, the preparations for it had taken more than three years, Israeli officials said.

But the distance between the current government’s threats and its ability to carry them out has provoked criticism of the former prime minister, Benjamin Netanyahu, who led Israel’s government until last June and was a dogged advocate for a harsher approach to Iran.

Since 2015, training for a strike on Iran had slowed, a senior Israeli military official said, as the defense establishment focused on confrontations with militias in Lebanon, Syria and Gaza.

In 2017, the Israeli Air Force determined it needed to replace its refueling planes, but Mr. Netanyahu’s government did not order them until last March.

And another senior military official said the army had asked Mr. Netanyahu since 2019 for extra funds to improve Israel’s ability to attack Iran, but was rebuffed.

In a statement, Mr. Netanyahu’s office said the opposite was true, that it was Mr. Netanyahu who pushed for more resources and energy on a strike on Iran while the military chiefs insisted on spending most of their budget on other issues and slowed down preparations to strike Iran.

“Were it not for the political, operational and budgetary actions led by Prime Minister Netanyahu over the past decade, Iran would have long had an arsenal of nuclear weapons,” the statement added.

Whether or not Mr. Netanyahu restricted the funding, experts have said that the money under discussion would not have significantly changed the army’s ability to attack Iran.

“You can always improve — buying more refueling airplanes, newer ones, bigger loads of fuel,” Mr. Shafir said. But even with these improvements and a superior air force, he said, Israeli airstrikes would not end Iran’s nuclear program.

They would likely, however, set the region on fire.

Ronen Bergman reported from Tel Aviv, and Patrick Kingsley from Jerusalem. Myra Noveck contributed reporting from Jerusalem, and Rawan Sheikh Ahmad from Haifa, Israel.