JERUSALEM (AP) — For more than a decade, Israel has systematically built up its military specifically for a possible strike on Iranian nuclear facilities. It has sent its air force on long-distance training missions, procured American-made "bunker-busting" bombs and bolstered its missile defenses.

Prime Minister Benjamin Netanyahu's threats to strike Iran, voiced last week during a high-profile visit to the White House, were not empty bluster. Although a unilateral Israeli attack would probably not destroy Iran's nuclear program, it appears capable, at least for now, of inflicting a serious blow.

"If Israel attacks, the intention is more to send a message of determination, a political message instead of a tactical move," said Yiftah Shapir, a former Israeli air force officer who is now a military analyst at the INSS think tank in Tel Aviv.

Israel, along with the United States and other Western countries, believes Iran has taken key steps toward developing nuclear weapons. The U.N.'s nuclear watchdog agency has cited this concern in reports, but notes its inspectors have found no direct evidence that Iran is moving toward an atomic weapon.

Israeli leaders, however, argue that time is quickly running out. They have grown increasingly vocal in their calls for tough concerted international action against Iran while stressing they are prepared to act alone if necessary.

Israeli defense officials believe Iran is capable of producing highly enriched weapons-grade uranium within six months. After that, it would require another year or two to develop a means of delivering a nuclear bomb, they predict.

But Israel believes the window to act will close much sooner than that. Officials say in the coming months Iran will have moved enough of its nuclear facilities underground and out of reach of conventional airpower, and that the world will be powerless to stop it. Defense Minister Ehud Barak calls this the "zone of immunity."

Defense officials acknowledge that plans to go after Iran have been in the works for years, with the air force expected to take the lead in what would be an extremely complicated operation. The officials spoke on condition of anonymity because they were discussing sensitive military deliberations.

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Israel has a total of 300 warplanes, but about 100 front-line planes would participate in the mission, officials suggest. They would include attack aircraft as well as others used to escort, target enemy warplanes and anti-aircraft batteries and provide support like communications and search and rescue.

The most powerful is the squadron of 24 F15i warplanes, American-made aircraft capable of carrying heavy payloads that could include 5,000-pound (2,200 kilogram), laser-guided GBU-28 bombs purchased from the U.S. These "bunker-busting" bombs would be at the heart of any operation.

In addition, Israel has four squadrons, or about 100, F-16i warplanes. These planes are more nimble in the air, capable of attacking ground targets but also ideal for escorting the heavier attacking aircraft. The air force also has developed long-range unmanned drones that can provide intelligence, communications and other support in any mission.

Experts believe that some of the Israeli warplanes, even F16s with upgraded fuel tanks, could not make the round trip without refueling in flight — depending on the route as well as the weight of their payload. Israel, which has eight tanker planes, can refuel an airplane in flight in a matter of minutes, though it's unclear where the task would take place since much of the airspace in the region is hostile.

There is precedent: Israeli warplanes destroyed an unfinished Iraqi nuclear reactor in 1981, and did the same thing to a nascent reactor in Syria in 2007. But an operation in Iran would be far more difficult — complicated by distance, stronger Iranian defenses and the Iranian strategy of scattering its nuclear installations in underground locations.

The Israeli air force has carried out a series of long-distance training runs that could serve as models for striking Iran. In 2008, 100 jets participated in a drill in Greece. The air force has carried out similar drills more recently with both Greece and Italy, officials say.

Probable targets in Iran, including the Natanz and Fordo enrichment facilities south of Tehran, lie some 1,000 miles (1,600 kilometers) from Israel.

Shafir, the former air force officer, said planners would need to choose among three likely flight paths, all of which carry grave risks.

The shortest, most direct flight would be to cross over neighboring Jordan and through Iraq.

Neither country has the capability to stop Israeli warplanes from crossing through its airspace. But this would deeply embarrass them.

Such an operation would raise the likelihood of a diplomatic spat with Jordan, Israel's closest ally in the Arab world, and potentially expose it to Iranian retaliation. Jordanian officials refused to comment on how the government would react if Israel uses its airspace.

A second route would be to fly south and through Saudi Arabia. The Saudis have no relations with Israel, and while they feel deeply threatened by a nuclear Iran, any signs of cooperation with the Jewish state would unleash fierce criticism throughout the Arab world. The Saudis would also be an easy target for an Iranian counter-strike.

The last possibility would be crossing through Turkey, as Israel illicitly did in the 2007 airstrike in Syria. But Turkey is believed to have upgraded its radar systems since then, and Israel's relations with Turkey, once a close ally, have deteriorated.

A Turkish official said it was "out of the question" for Israel to use Turkish airspace. He said the jets would be "brought down" if Israel attempted to use the airspace without permission. He spoke on condition of anonymity because he was not authorized to comment publicly on the matter.

Once Israeli planes reach Iran, they would come under fire from Iranian air-defense systems and warplanes. Israeli officials say they take these threats seriously, but believe Israel's superior firepower and radar-jamming technology would allow them to perform the mission.

Iran's air attack capabilities depend heavily on domestically modified versions of long-outdated warplanes, including former Soviet MiGs and American F14A Tomcats from the 1970s.

Iran is also believed to possess retooled versions of Russia's state-of-the-art S-300 anti-aircraft missiles, as well as advanced Chinese radar systems. Russia has held up an official sale of S-300 defenses for five years, citing technical glitches.

Outside experts say Iranian capabilities, particularly homegrown technologies, are limited.

The biggest challenge to Israel may be the limits of its firepower. Iran's main uranium-enrichment facility at Natanz is believed to be about six meters (25 feet) underground and protected by two concrete walls.

This would stretch the capabilities of Israel's arsenal of bunker busters and explains why the Israelis would much prefer that the U.S. take the lead in an operation. The U.S. has forces near Iran in the Gulf and possesses bunker busters even more powerful than Israel's.

Iran has also been shifting its enrichment operations to the far more fortified Fordo site, dug 300 feet (90 meters) into a mountain south of Tehran. Further complicating the task, Israeli officials say, Iran uses special Russian-made nets that conceal the facilities and distort the detection of Western spycraft.

Iran has threatened to retaliate and has developed sophisticated Shahab missiles capable of striking the Jewish state. It also could encourage its local proxies, Hezbollah in Lebanon and Hamas in the Gaza Strip, to unleash their arsenals of tens of thousands of rockets.

Hezbollah has not said what it would do, while Hamas has signaled it does not want to get dragged into an Israel-Iran war.

Nonetheless, Israel has developed a series of air-defense systems for the various threats. It has begun testing the third generation of its Arrow system, designed to shoot down incoming missiles from more distant origins like Iran. It also has deployed its "Iron Dome" rocket defense system, which has successfully shot down nearly 80 percent of incoming rockets from Gaza in a new round of fighting in recent days.

Many experts believe Iran would retaliate against American targets in the Gulf, as well as U.S. allies like Saudi Arabia for their perceived support of an Israeli strike.

Any unilateral strike would likely also draw fierce international criticism. That means an Israeli operation would have to be short-lived, perhaps a one-time attack, and not a sustained air campaign.

Scott Johnson, an analyst at the IHF Jane's military research firm, said that given these limitations, Israel would at best set back, but not neutralize, the Iranian program. Success, he added, would depend on the effectiveness of the bunker busters.

Danny Yatom, a former director of Israel's Mossad spy agency, said even if Israel cannot destroy Iran's nuclear program altogether, a serious disruption would be enough.

"This might delay the appearance of the bomb by many years," he said.

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Brian Murphy in Dubai, United Arab Emirates, Suzan Fraser in Ankara, Turkey and Jamal Halaby in Amman, Jordan, contributed to this report.

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AP Interactive: Iran nuclear sites http://apne.ws/zEfdpT