The Bush administration has sent undercover forces into Iran, and has stepped up secret planning for a possible major air attack on the country, according to the renowned US investigative journalist Seymour Hersh.
While publicly advocating diplomacy to stop Iran from pursuing a nuclear weapon, Hersh reports in the next issue of The New Yorker magazine that "there is a growing conviction among members of the United States military, and in the international community, that President Bush's ultimate goal in the nuclear confrontation with Iran is regime change".
One former senior intelligence official is quoted as saying that Mr Bush and others in the White House have come to view Iran's president, Mahmoud Ahmadinejad, as a potential "Adolf Hitler".
According to a senior Pentagon adviser on the "war on terror", "this White House believes that the only way to solve the problem is to change the power structure in Iran, and that means war".
The danger, he adds, is that "it also reinforces the belief inside Iran that the only way to defend the country is to have a nuclear capability".
One option under consideration, Mr Hersh reports, involves the possible use of a B61 nuclear "bunker-buster" bomb against Iran's main centrifuge plant, at Natanz.
Last week the Federation of American Scientists alleged that a weapons test to be carried out in the Nevada desert in June was designed to simulate the effects of just such a bomb.
Conventional explosives would be used, it said, for "a low-yield nuclear weapon ground shock simulation against an underground target".
The US Defence Threat Reduction Agency told The Independent on Sunday that the test, codenamed "Divine Strake", was intended "to assess the capability of computer codes" to predict the effects of the explosion.
The experiment aimed to improve "warfighters' confidence in their ability to plan to defeat hardened and deeply buried targets".
It did not refer to tactical nuclear weapons like the B61.
According to Mr Hersh, some officials are shocked at what they describe as "operational" planning which goes far beyond the usual work on hypothetical scenarios.
One former defence official is quoted as saying the planning was based on a belief that "a sustained bombing campaign in Iran will humiliate the religious leadership and lead the public to rise up and overthrow the government".
Some operations, apparently aimed in part at intimidating Iran, are already under way, Mr Hersh reports, including "simulated nuclear-weapons delivery missions" by US navy aircraft operating from carriers.
Undercover units are also said to be working with ethnic minorities in Iran, including the Kurds, Baluchis and Azeris.
While one goal was to have "eyes on the ground", the broader aim was to "encourage ethnic tensions" and undermine the regime.
Britain and other European states support the need for a military option to deter Iran from developing nuclear weapons, The New Yorker article says, but want nothing to do with regime change.
"The Brits think this is a very bad idea, but they're really worried we're going to do it," Flynt Leverett, a former member of the US National Security Council, is quoted as saying.
Critics of military action against Iran point out that it would convulse world oil markets and could lead to retaliation in Iraq.
Mr Hersh says he was told by a Pentagon adviser that the southern half of Iraq, where Britain's 8,000 troops are based, would "light up like a candle" in the wake of any strike on Iran, while a general said that, despite the British presence, "the Iranians could take Basra with 10 mullahs and a sound truck".
The greatest disquiet within the military is said to be over the possibility of using nuclear weapons against Iran.
Some planners argue that it would be impossible to be certain that underground facilities such as those at Natanz had been completely destroyed unless a nuclear "bunker-buster" was used.
Mr Hersh says he was told by a former senior intelligence official that some officers had talked about resigning after an attempt to remove the nuclear option from the war plans failed.
The Pentagon adviser warns, as do many others, that bombing Iran could provoke "a chain reaction" of attacks on American facilities and citizens throughout the world.
"What will 1.2 billion Muslims think the day we attack Iran?" he asks.
Mr Hersh reports that the White House refused to comment on military planning, but insisted, as did the Pentagon, that a diplomatic solution was being sought with Iran.
The CIA said there were "inaccuracies" in his account, but would not specify them.