DEMOCRATS accused the Bush Administration yesterday of risking a new arms race after the Senate backed a Pentagon plan to research a new generation of “mini” battlefield nuclear weapons.

In one of the stormiest debates in the Senate chamber since President Bush took office, Democrats lost a fight to keep a ten-year Clinton-era ban on the research and development of low-yield nuclear weapons.

Although the chamber later voted that Mr Bush must obtain congressional approval before the building of any new weapons, Democrats lined up to denounce the research.

They claim that it is the first step towards a new type of nuclear arsenal and undermines US efforts to persuade other countries not to develop their own nuclear weapons.

Donald Rumsfeld, the Defence Secretary, said that there were no existing plans to build smaller nuclear weapons, but the Administration has been manoeuvring for months to update the US nuclear capability. The underlying thinking, pushed by Pentagon hawks, is that the US Cold War arsenal of long-range and medium-range ballistic missiles has lost its deterrence value because the US is so unlikely to use them.

Smaller, low-yield weapons would offer a more credible deterrent, proponents argue, because they are weapons that the Pentagon may actually use.

Mr Rumsfeld said ahead of the Senate debate that nuclear weapons of five kilotons or less could be more effective than conventional bombs in strikes against deeply buried chemical and biological arsenals. The bomb dropped on the Japanese city of Hiroshima in the Second World War, which killed 100,000 people, was 13 kilotons.

“We’re going to be looking at a variety of ways, conceivably, to develop the ability to reach a deeply buried target,” Mr Rumsfeld said. “Many of the things you study, you never pursue.”

“Just a study? Baloney!” Diane Feinstein, a California Democrat, shouted during the Senate debate. “Does anyone really believe that?” But in a 51-43 vote, the Senate defeated her amendment to keep a ban on the research and development of the weapons.

The debate came as the Republican-controlled House and Senate prepared to pass a huge defence-spending Bill authorising more than $400 billion (£275 billion) for the Pentagon in the fiscal year 2004. Passage of the Bill will give the Pentagon billions of extra dollars to develop the previously outlawed nuclear bombs and to breathe new life into America’s nuclear infrastructure, which President Bush views as a vital part of US strategy.

The Bill would also require the US Energy Department to be ready to resume underground nuclear tests within 18 months. America suspended nuclear weapons-testing 11 years ago. Restarting underground tests would be in breach of the 1996 Nuclear Test Ban Treaty, but Mr Bush has the power to rescind the US as a signatory, without congressional approval.

“This Bill is a declaration that the United States is prepared to launch a nuclear arms race in the world again,” Senator Richard Durbin, an Illinois Democrat, said. His colleagues also say that the “mini-nukes” would blur the line between nuclear and conventional weapons, making their use more likely.

“This issue is as clear as any issue gets,” Ted Kennedy, the veteran Massachusetts senator, said. “You’re either for nuclear war or you’re not. Either you want to make it easier to start using nuclear weapons or you don’t.”

Daryl Kimball, executive director of the Washington-based Arms Control Centre, said: “These weapons are devastating. No matter how much smaller and deeper we can make these weapons go, there will be tremendous collateral damage and contamination.”

Advocates of lifting the ban say that the weapons could target enemies more precisely while limiting civilian deaths. They also argue that they could be a safer way to eliminate stockpiles of illegal weapons by reducing widespread chemical and biological contamination that might follow a less destructive strike.

“We should continue with the basic theme that we’re not going to impose a ban on this nation with respect to this system or any other system which may be needed in defence of this nation,” John Warner, a Republican senator, said.

The Administration also unveiled an expanded missile defence programme that sought to broaden its appeal to allies. It no longer referred to “national” missile defence but placed new emphasis in protecting areas far from US borders — a reference to the threat posed by North Korea’s nuclear programme. Its release was partly linked to the arrival here today of Junichiro Koizumi, Japan’s Prime Minister.

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