Only 22 inches (56 centimeters) long and eight inches (20 cm) in diameter, they are housed in needle-shaped nose cones that fit atop the nuclear warhead of a Minuteman intercontinental ballistic missile, officials said.

"It's very sensitive technology because it is essentially the brain of the re-entry vehicle," said Hans Kristensen, an expert on nuclear weapons at the Federation of American Scientists.

"As it comes toward the Earth, it determines when the bomb goes off and that it goes off at the right height and at the right yield," he said.

At exactly the right moment, an electrical signal from the fuse activates detonators on chemical explosives surrounding the sphere that contains the plutonium trigger of a thermonuclear weapon.

"That has to be very, very, very precise, because if it's not then you would have some sort of a squish effect where the plutonium would be misformed by the explosion," Kristensen said.

The chemical explosives ignite the plutonium detonation, releasing "this enormous bombardment of neutrons flying everywhere that bounce off refelectors inside the weapon," he said.

The reflectors concentrate the neutrons to ignite hydrogen in a secondary, far more powerful explosion.

Senior Pentagon officials said the fuses and their nose cone assemblies were mistaken for helicopter batteries and shipped to Taiwan in late 2006.

The error went undiscovered until last week, when the US military scrambled to recover them.

The officials said they used 1960s technology, and suggested that they were little different from fuses used in artillery shells and other weapons.

"The specific manufacturing of this is done to be mated specifically with this weapons package," said Ryan Henry, a senior Pentagon official.

"So you would not be able to use this in any other weapons system, nuclear or non-nuclear. But the mechanism itself is common to many, many different weapons," he said.

But Kristensen said these fuses are unique and their nose cone assemblies are also the product of years of costly development.

"And for any country that develops such technology, it is a hugely important technology," he said.

"Because this phase of the re-entry vehicle as it passes through the atmosphere at a very, very high speed and under enormous stress and turbulence you have to be able to set the height of burst very accurately if you want to have maximum capability out of your warhead," he said.

"So for a country like China, that is trying to development more capable systems, that would be very important material to get. And for any country that is even lower on the nuclear threshold scale, having not quite gotten there, it would be potentially even more important," he said.