But the administration official said the "adjustments" in the U.S. position narrow any contingencies for a nuclear strike.
Protection from such danger is by itself worth the vast resources which a nuclear strike against the comet will require.
The convention bans such systems under the belief that a country would not launch a nuclear strike if it were unable to protect itself against retaliation.
The second world war saw mass murder (in the Holocaust, notably) or wilful killing (the nuclear strike on Hiroshima, the firebombing of Dresden, the siege of Stalingrad) of tens of millions of non-combatants.
And that has added to the frustration over, for example, Mr Putin's backing for Syria's government or a senior Russian general's statement that the country did not rule out the possibility of a nuclear first strike against missile-defence sites.
But despite Mr. Netanyahu's pledge of revenge, it is unlikely that Israel would use Wednesday's attack as grounds to strike Iranian nuclear targets, a move opposed by the U.S. and likely to trigger a regional war.
Almost every assessment of a possible Israeli strike on Iran's nuclear installations has assumed that Israel will use its air force to strike.
The first is that although the West is safe in the sense that the existential threat of invasion by a malevolent foreign power or a first strike by a nuclear-armed aggressor is now vanishingly small, the rise of jihadist terrorism has left people feeling vulnerable.
Moreover, he predicts that "the UK government's strong commitment to nuclear power and to energy security means the 'strike price' will be set at a fair level to secure investment in the nuclear industry".
In 1981, Israel launched an air strike on an Iraqi nuclear reactor destroying it completely.
In press interviews, Dagan stated that it would be disastrous for Israel to strike Iran's nuclear installations.
Gabi Ashkenazi - who strangely received a nice medal from Mullen two years ago - is the main opponent of an Israeli military strike against Iran's nuclear installations.
Diskin, a widely respected former security official, also took the two Israeli leaders to task for mis-characterizing the possible effectiveness of a military strike against Iran and its nuclear program.
The wider world will examine these results for clues about Israel's future attitude towards peace talks with the Palestinians or the possibility of a pre-emptive strike on Iran's nuclear facilities.
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If its domestic stockpile could in this way be kept below a bomb's worth, this would also ease pressure in Israel for a military strike on Iran's nuclear facilities and so buy yet more time for talks.
Over the past fortnight, a number of articles have appeared in Israeli newspapers claiming that the prime minister, Binyamin Netanyahu, and the defence minister, Ehud Barak, have dusted off long-standing plans for a pre-emptive strike on Iran's nuclear facilities.
If they do not, their own military men will spend an increasing amount of time worrying about the danger of losing all their nuclear might to an enemy's first strike.
In an interview with CNN, Oren said that Israel is "far from even contemplating" a military strike against the Islamic republic's nuclear installations.
The doves agree with Netanyahu that a limited Israeli strike is better than the alternative of a nuclear-armed Iran.
Mr. Panetta and former Defense Secretary Robert Gates have argued that a military strike would at best delay Iran's nuclear development for a few years.
Despite preparatory work on the site being complete, EDF is waiting to agree the strike price with the government - for the nuclear power generated - before construction can begin.
Thus, the U.S. has sought to organize its nuclear forces so that no enemy can launch a disarming first strike, mainly by fielding three distinctly different types of weapons: manned bombers, land-based intercontinental ballistic missiles in hardened silos, and sea-based intercontinental ballistic missiles on stealthy Trident submarines.
Is it against a nuclear-armed adversary or one with only limited long-range strike capabilities?
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During the Cold War, when the United States faced the threat of a nuclear attack, U.S. administrations made clear that any strike would prompt an all-out retaliation.
Workers at several Russian nuclear centres - some of them in closed military towns - went on strike on Monday demanding payment of huge wage backlogs by the federal authorities, Russian television reported.
They are the only nations that both possess nuclear weapons and are large enough, in theory, to absorb a first strike and live to retaliate.
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He said he was not calling for "the instant abandonment of nuclear power" but "just for a pause, for the referral of the strike price negotiation to the Public Accounts Committee, other select committees or to an independent panel of experts".
As nuclear arsenals grew, each of the two main sides in the Cold War gained second strike capability, rendering a surprise attack moot.
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Talks are continuing to try to avert the first ever strike at the firm that builds warheads for Trident, the UK's nuclear weapons system.
Tensions increased between nuclear rivals India and Pakistan following the November 26 attacks in Mumbai, where militants launched a coordinated strike against luxury hotels, a Jewish center and other targets.
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