Scientists in the United States claim to have devised a novel technique to test the viability of nuclear warheads, a tool that could be useful for disarmament inspectors.

The method uses a neutron scanner to confirm whether a nuclear warhead is what its owners say it is, without divulging any classified secrets about the device -- a major obstacle in weapons verification, they said on Wednesday.

The technique, currently in the early stages of testing, should be able to test whether rogue states or groups claiming to have a nuclear bomb are telling the truth.

It could also be a useful tool in the programme to dismantle US and Russian nuclear warheads under the New Strategic Arms Reduction Treaty (New START), they said.

"The goal is to prove with as high confidence as required that an object is a true nuclear warhead while learning nothing about the materials and design of the warhead itself," said Robert Goldston, a professor of astrophysics at Princeton University, New Jersey.

Weapons inspectors already have an array of diagnostic instruments on hand, but using them can be a problem in itself.

Gamma-ray spectroscopy, for instance, can reveal whether there is sufficient plutonium 239 to make a bomb, but measuring this would reveal warhead-design information that could help weapons proliferation.

Other procedures are likely to require opening up the warhead to verify it -- a process that is long, complex and laden with suspicions that this is an attempt to spy on or tamper with secret material.

To get around this, Goldston's team conceived of an approach called "zero knowledge," inspired in part by software designed to check computer passwords safely.

It entails aiming a high energy beam of neutrons through the warhead, rather like an X-ray.

The tally of neutrons detected on the other side of the warhead thus provides a signature of the contents.

This signature has to match a signature provided by the host to be confirmed as a bomb.

Because the host preloads his own signature into the detectors, the inspector requires no access to secret data or material, hence "zero knowledge" of what lies in the box.

The idea, reported in the science journal Nature, is still in its infancy and problems have to be ironed out, the scientists admit.

But it has secured funding of $3.5 million (2.57 million euros) from the US National Nuclear Security Administration to take it further at the Princeton Plasma Physics Laboratory.

The researchers are using a harmless dummy made of polystyrene and tungsten, about the same size and weight as a warhead, to test the neutron scan.