Congress has stalled Pentagon plans to put conventional warheads on inter-continental missiles for use in Washington's "war on terror", out of concern that they could trigger a nuclear war.
The defence department is seeking $127m (£68m) for the conversion of submarine-based missiles as part of its Global Strike project, aimed at giving Washington the option of acting fast, pre-emptively and from great distances against targets that might threaten the US.
The goal would be to destroy a fleeting target, such as a weapon being assembled or a meeting of terrorist leaders, anywhere in the world within an hour of intelligence reaching the US of their location.
But Congress has held back $32m of the funds until the secretary of defence, Donald Rumsfeld, and the secretary of state, Condoleezza Rice, present a plan to eliminate the risk that other countries mistake the launch of the Trident D-5 missile for a nuclear attack and respond.
"There is great concern this could be destabilising in terms of deterrence and nuclear policy," Jack Reed, a Democrat on the senate armed services committee, told the New York Times. "It would be hard to determine if a missile coming out of a Trident submarine is conventional or nuclear."
"It's a valid concern - you never want someone to think you're launching a nuke when you're not," Colonel Richard Patenaude, a deterrence and air strike strategist told Inside the Air Force, a defence newsletter. "But I think it's a manageable problem, and a lot of others do too."
Pentagon planners are confident a system can be worked out with other nuclear powers to reassure them that a Trident launch is conventional and not aimed at them. In 2000 the US and Russia agreed to establish a joint data exchange centre to share information about ballistic missile launches.
The head of the US Strategic Command, General James Cartwright, who has oversight over the nuclear and non-nuclear elements of Global Strike, said the US informed China of any test launches.
"We don't have a treaty, but we tell them so that they know," Gen Cartwright recently told Japanese journalists. He said he hoped China and other nations would join the US-Russian data exchange centre.
The Global Strike programme was part of the US Nuclear Posture Review, submitted by the Pentagon at the end of 2001, which recommended the restructuring of US strategic defences away from its cold war arsenal towards smaller weapons, nuclear and non-nuclear, which could be used against terrorists or rogue states believed to be planning an attack using weapons of mass destruction.
The review has been criticised for blurring the line between nuclear and conventional weapons, and Congress has blocked Pentagon plans to build a new generation of nuclear bunker-busters and "mini-nukes". Critics also question whether a US president could be so sure of intelligence to order the launch of a long-range ballistic missile.
Guardian Unlimited © Guardian Newspapers Limited 2006