JERUSALEM (Reuters) - Deeply concerned as it is by the risk of a nuclear-armed Iran, Israel has never even hinted at using atomic weapons to forestall the perceived threat.
But now a respected Washington think tank has said that low-radioactive yield "tactical" nuclear warheads would be one way for the Israelis to destroy Iranian uranium enrichment plants in remote, dug-in fortifications.
Despite the 65-year-old taboo against carrying out -- or, for that matter, mooting -- nuclear strikes, the Centre for Strategic and International Studies (CSIS) says in a new report that "some believe that nuclear weapons are the only weapons that can destroy targets deep underground or in tunnels."
But other independent experts are on record warning that such a scenario is based on the "myth" of a clean atomic attack and would be too politically hazardous to justify.
In their study titled "Options in Dealing with Iran's Nuclear Program," CSIS analysts Abdullah Toukan and Anthony Cordesman envisage the possibility of Israel "using these warheads as a substitute for conventional weapons" given the difficulty its jets would face in reaching Iran for anything more than a one-off sortie.
Ballistic missiles or submarine-launched cruise missiles could serve for Israeli tactical nuclear strikes without interference from Iranian air defences, the 208-page report says. "Earth-penetrator" warheads would produce most damage.
Israel is widely assumed to have the Middle East's sole atomic arsenal. Israeli leaders do not comment on this capability other than to underscore its deterrent role; President Shimon Peres has said repeatedly that "Israel will not be the first to introduce nuclear weapons to the region."
A veteran Israeli defence official, speaking on condition of anonymity, said pre-emptive nuclear strikes were foreign to the national doctrine: "Such weapons exist so as not to be used."
A fixture of NATO and Soviet arsenals, tactical nuclear weapons are designed to deliver focussed devastation with less contamination than city-killing bombs like those the United States dropped on Japan to end World War Two.
That damage containment would, in theory, off-set diplomatic fallout for whichever country were to use such arms on a foe.
There has been speculation that the United States -- which, like Israel, has not ruled out military force to deny Iran atomic arms -- could itself resort to tactical nuclear strikes.
The Pentagon's 2002 Nuclear Posture Review, which was leaked to the media, spoke of the need to develop new "mini-nukes" for defeating bunker systems. The review cited Iran among potential enemies that might eventually warrant a U.S. nuclear deployment.
Yet Toukan and Cordesman think it "very unlikely that any U.S. president would authorise the use of such nuclear weapons, or even allow ... a strong ally such as Israel to use them, unless another country had used nuclear weapons against the U.S. and its allies."
They say the United States would be central to any diplomatic solution to the Iranian standoff and is the only country that could launch a successful military strike on Iran.
International experts who contributed essays to the 2003 book "Tactical Nuclear Weapons" mostly shied from hawkishness.
"Who could predict what might happen next if (the) taboo on the use of nuclear weapons were to be broken?" wrote former CIA director Stansfield Turner. "Getting tactical nuclear weapons under control, rather than attesting to their use by building new ones, should be our goal."
Princeton University physicist Robert Nelson assailed the idea that tactical nuclear weapons, detonated below ground, would pose tolerable risks for civilians and the environment.
"This is a dangerous myth. In fact, shallow buried nuclear explosions produce far more local fallout than air or surface explosions of the same yield," he argued.
Sam Gardiner, a retired U.S. air force colonel who runs wargames for various Washington agencies, said an Israeli decision on using non-conventional weapons against Iran would come down to how far its nuclear programme was to be retarded.
Israel supports efforts by world powers to rein in Iran -- which denies seeking the bomb -- through sanctions, and some experts say any pre-emptive Israeli strike would aim to jolt international diplomats into finally knuckling down on Tehran.
"If a 3-to-5 year delay were the Israeli objective, I expect it would drive their target people to say the only way it could be done is with tactical nuclear weapons," Gardiner said.
"I expect the Israeli objective to be more like a year. That is doable without tactical nuclear weapons."
(Writing by Dan Williams; Editing by William Maclean)