Officials said the fuze was only fifty-six centimeters long and twenty centimeters in diameter, and was fitted into the needle nose cone at the front end of the militia intercontinental ballistic missile.

"This is a very sensitive technology, because it is essentially the brain of the re-entry vehicle," said Christensen, a nuclear scientist at the American Association of scientists.

He said, "when the missile approaches the earth, the fuze decides when the bomb will explode and explode at the right height and the right equivalent."

U.S. officials said the fuzes used the technology of the 60s of last century, meaning they were very different from fuzes used by artillery shells and other weapons.

Henry, a senior US defense official, said the fuze was specially designed for militia missiles and could not be used in other nuclear or non nuclear weapon systems, but the mechanism itself is the same for many, many different kinds of weapons.

Christensen said that this technology is very important for China and other nuclear weapon states that want to develop more powerful power.

He said: "when the missile enters the atmosphere again, it is very, very fast, and bears great pressure and turbulence. If we want to maximize the power of the warhead, we must set the height of the explosion very accurately."