At the same time the administration has pursued advanced nuclear weapons technology of its own, involving smaller bombs designed for bunker busting and, potentially, for tactical battlefield use.
Critics of the programme say the US is degrading its stated desire to curb global nuclear proliferation, seeking to enhance its arsenal while discouraging other nations from developing theirs.
"Even as we try to persuade North Korea to pull back from the brink, even as we try to persuade Iran to end its nuclear weapons programme, the Bush administration now wants to escalate the nuclear threat by developing two new kinds of nuclear weapons for the United States," Democratic Senator Edward Kennedy said recently during a speech.
Kennedy called the Bush policy "a shameful double standard".
"Our goal is to prevent nuclear proliferation," he said. "How does it help for us to start developing a new generation of nuclear weapons?"
However, supporters of the programme, such as Republican Senator Wayne Allard, say the US must "maintain a credible [nuclear] deterrent," in order to meet the threat posed by rogue nations and terrorist groups.
"In fact, if the United States does not show that it is serious about ensuring the viability of our entire military capability, including our weapons of last resort, we might not be able to dissuade potential adversaries from developing weapons of mass destruction and deter those adversaries from using those weapons they already have," Allard said during a recent speech.
The US delegation at talks on the
North Korean nuclear issue, Beijing
The two programmes in question are in the research stage and still years away from development and production.
In its 2005 fiscal budget proposal, the Bush administration is asking Congress for $27m to research the Robust Nuclear Earth Penetrator, a programme designed to modify existing types of nuclear weapons for the purpose of destroying deep bunkers. It also seeks $9m for research on the Advanced Concepts Initiative, which could create low-yield "mini-nukes".
Even as it proposes a five year, $485m plan for research and development of these programmes, the Bush administration has castigated Iran and North Korea for pursuing nuclear technology.
Many arms control experts say this represents a diplomatic contradiction and could diminish the ability of the US to persuade smaller countries to abandon efforts to become nuclear powers.
"It would help that argument [of non-proliferation] to not be pursuing these types of weapons that could destroy certain targets within these countries," said Daryl Kimball, president of the Arms Control Association, a policy group in Washington.
"The US pursuit of a new nuclear weapon is remarkably stupid and short-sighted politically"
Joseph Cirincione, director of non-proliferation projects, Carnegie Endowment for International Peace
Some analysts worry the administration's policy could convince countries such as South Korea, Taiwan, Saudi Arabia and Egypt that nuclear weapons are necessary and unavoidable in the 21st century.
"I worry more about the fence-sitters," said Michael Levi, an expert on weapons of mass destruction at the Brookings Institution.
Others say they worry more that these new programmes could ultimately increase the likelihood that nuclear weapons will be used in war zones.
"The US pursuit of a new nuclear weapon is remarkably stupid and short-sighted politically," said Joseph Cirincione, director of non-proliferation projects at the Carnegie Endowment for International Peace. "If the United States pursues this policy it will be erasing the wall between conventional and nuclear weapons."
Cirincione cited the most recent US Nuclear Posture Review, submitted to Congress in late 2001, which stated that a "new mix" of nuclear capabilities "is required for the diverse set of potential adversaries and unexpected threats the United States may confront in the coming decades."
Some experts interpret this statement as an indication that the administration wants to develop a viable option for the use of low-yield nuclear weapons on the battlefield.
"It will be more likely that nuclear weapons will be used in combat," Cirincione said.
Although Levi said he had not seen any evidence that the Bush administration is trying to develop battlefield nuclear weapons, he worried that the rumours surrounding the Advanced Concepts Initiative could give other countries the wrong impression.
"As soon as you provide even the smallest impression that something is even possible, then the impressions go beyond your control," he said.
The US could encourage others
to develop nuclear weapons
Despite predictions that they could be safely used to destroy weapons caches hidden deep underground, some experts say the so-called bunker-buster bombs pose serious dangers to the surrounding environment.
By modifying the casing around two types of existing warheads, it is believed by some that the warheads could then penetrate deep enough into the earth that the resulting shockwave would be contained.
Kimball, however, said the size of the Robust Nuclear Earth Penetrator, with a blast yield of likely more than 100 kilotons, would create an explosion so big that it could cause extensive above-ground damage. By comparison, the bomb dropped on Hiroshima during the second world war was only 10 kilotons.
"To contain the fallout of a relatively small, five-kiloton nuclear bomb, it would have to be detonated about 350ft underground - nearly 10 times the depth that existing materials and force capabilities allow," Kimball wrote in a May article for Arms Control Today.
"Even if smaller weapons were used against suspected chemical or biological weapons sites, errors in intelligence and targeting could disperse rather than destroy deadly material."
Although Congress allocated $21m for research of these programmes in 2004, a majority of the US public opposes the development of new types of nuclear weapons, according to a nationwide poll conducted in June by the Program on International Attitudes at the University of Maryland.
Roughly 59% said it was not necessary for the United States to develop new nuclear weapons beyond those it already has.
If President George Bush wins a second term in November, he would still need approval from Congress to begin development and production of the new weapons. If that happens, Kimball estimates that they could potentially be ready by 2010.