The Bush administration is pushing ahead with plans to research, and perhaps build, new kinds of nuclear weapons, angering critics who say that this will encourage global proliferation.

The programme was outlined by energy secretary Spencer Abraham and Linton Brooks, head of the National Nuclear Security Administration, at congressional hearings over the past two weeks. It would see some $500 million spent on advanced concepts for nuclear weapons over the next five years. This would pay for research and development of a nuclear-tipped cruise missile with advanced navigation capabilities and a weapon known as a bunker buster, which could strike targets buried deep underground.

Leading Democrats lined up to denounce the plan, saying that the energy department has other nuclear-weapons priorities that seem to be suffering at the expense of the new programme. “You're rushing ahead with new nuclear weapons including mini-nukes and nuclear bunker busters,” said Senator Edward Kennedy (Democrat, Massachusetts). “But you're cutting funds for nuclear security.”

Since the Bush administration unveiled its nuclear doctrine in 2001, officials have advocated the development of low-yield nuclear weapons that might be used against hardened or deeply buried targets without causing massive civilian casualties. They have also argued that working on such weapons will strengthen the skills at the nuclear-weapons labs — Lawrence Livermore in California, and Los Alamos and Sandia in New Mexico (see Nature 415, 945–946; 2002). But critics counter that the availability of the weapons will increase the possibility that they will be used, by the United States or by others.

The president's budget for 2005 proposes that funding for research on “advanced concepts” should increase by 50% to $9 million. According to Abraham, much of that would go towards developing a cruise missile that could be retargeted after launch and would incorporate new safeguards against accidental detonation. Funding for research into a hardened warhead designed to destroy deeply buried targets would also increase from $7 million to $27 million.

But Democrats are angry at a five-year plan for these projects, contained in a five-year forecast of the budget, which suggests spending roughly $450 million for the development of the bunker-busting weapon between 2006 and 2009.

Abraham told the hearings that, for the moment, the weapons labs are carrying out only paper studies and have no plans to build or test new weapons. “All we are offering Congress is a cost assessment of what the programmes might be,” he said.

But some observers say that the five-year plan exposes the administration's real intentions. “Having budget numbers that extend through the development phase makes it harder for the administration to convince senators that this is just a research project,” says Michael Levi, a physicist at the Brookings Institution, a think-tank in Washington DC. “This indicates to me that the administration clearly intends to move forward,” adds Daryl Kimball, executive director of the Washington-based Arms Control Association, which advocates non-proliferation.

Last year, congressional opposition halved the administration's funding proposal for research into the bunker buster. This year, with an election at stake, it can expect an even tougher fight.