An Israeli pre-emptive strike on Iran's nuclear sites would amount to a roll of the dice against elusive targets, drawing in the United States if the gamble fell short, analysts and former US military officers say.

Iran's air defenses and aging fighters would be no match for Israel's high-tech aircraft and cyber warfare, but the outcome of a raid would largely hinge on intelligence and whether Tehran is able to hide key elements of its uranium enrichment network.

Israel's military has earned a reputation for lightning assaults that blindside their enemies, but the Iranian nuclear program presents a much more complicated task for Israel compared to previous raids that took out reactors in Iraq and Syria.

In 1981, Israeli fighters destroyed an Iraqi atomic reactor in Osirak without losing a plane and in 2007 the Israelis are widely believed to have knocked out a clandestine reactor in Syria.

But flying fighter jets 1,600 kilometers (1,000 miles) all the way to Iran would stretch Israel's limited supply of aerial refueling aircraft, while Tehran's dispersed, hidden nuclear sites -- including a facility dug into the side of a mountain -- present a daunting challenge.

"This is not a pinpoint, single target, one strike and it's over," said William Fallon, a retired navy admiral who led US Central Command in 2007-8.

An assault designed to delay Iran's nuclear work would be "very difficult" partly because the Iranians have "been pretty clever about distributing stuff," Fallon told an audience Thursday at the Center for Strategic and International Studies (CSIS).

Unlike in Syria or Iraq, Iran's nuclear program does not have a single vulnerable point that, if hit, would leave the project crippled for years.

Iran has developed the know-how to enrich uranium and a supporting industrial network to build centrifuges, which it has worked to conceal. As a result, Iran could potentially withstand a bombing campaign and still have enough centrifuge components and assembly plants to renew uranium enrichment work, analysts said.

"Equipment to make centrifuges can be moved relatively quickly. Centrifuge components that are finished can easily be moved and probably are. So what you may know one month, it may not be good the next month," said David Albright, president of the Institute for Science and International Security.

"If they (Iranians) feel that a strike is imminent they may move a lot of things... That's really the question, how much do the Israelis know about the program?" he told AFP.

Amid growing speculation that Israel may strike at Iran, the potential success of such a raid is the subject of intense debate in and outside the US military, but there is wide agreement that -- at best -- an attack would postpone and not paralyze Tehran's nuclear project.

Experts are assessing how long an Israeli attack could be sustained, how effective the bombing would be and what ultimate goal would be achieved.

Israel would need its entire fleet of about 125 high-end American-made fighter jets, including its 101 F-16Is and 24 F-15Is, to carry out such an ambitious raid, as well as all eight of its KC-707 refueling tankers, said Scott Johnson, an analyst at IHS Jane's defense consultants.

"They would use everything they have. Everything would be ready and everything would be in play," Johnson said.

Israeli fighter jets, using precision-guided bombs, cruise missiles and air-to-ground missiles, would likely be focusing on crucial links in the nuclear program, including an underground uranium enrichment center in Natanz, a centrifuge plant in Tehran and the recently revealed Fardow facility built into the side of mountain near Qom.

The Fardow site lies at least 80 meters (260 feet) underground and likely beyond the reach of even America's most powerful conventional bomb.

Israel would be under pressure to stage a quick attack, as any operation extending beyond one night could open the way to untenable military and political risks, said Daniel Levy, a senior fellow at the New America Foundation think tank.

"Both for technical and political reasons, the capacity for several repeat runs is a stretch," said Levy, who worked in Ehud Barak's Israeli government in 1999-2001.

Apart from triggering massive diplomatic fallout, a prolonged attack over days would remove any surprise element and Iran's air defenses would have a better chance of countering Israeli warplanes, Western officials said.

A job half-finished, with Iran's nuclear program bruised but not battered, could pull America into the fray with Tehran staging retaliatory strikes against Israel and attacks on US targets.

"When Israel games this scenario, strongly built into that is that America gets dragged into this," Levy said.