Doctors may be able to use a beam of sound waves to "trick" blood cells into clotting - avoiding the need for risky surgery.

The advance, reported in New Scientist magazine, uses the ability of the ultrasound to make blood platelets move around.

This makes them behave as if they were near an open wound, and start forming a clot.

Often the only solution to internal bleeding is an operation, but this carries additional risks for the patient, particularly in areas like the brain.

The researchers, from the University of Washington in Seattle, hope that the technique will be a precise way of stopping bleeding.

Their work is so far restricted to blood cells in a laboratory, but they hope to start work on humans as soon as possible.

When a blood vessel is burst, and bleeding starts, the platelets, a component of blood, link together across the wound and seal it.

Permanent damage

However, this takes time, and in areas like the brain, the blood loss can put pressure on delicate areas and cause permanent damage.

The ultrasound can be focused on a tiny area deep inside the body, and makes the blood move around more than normal, activating the platelets, and accelerating bleeding.

The ultrasound could potentially be used on wider areas of bleeding, for which operations are impractical.

And if gas-filled microbubbles called ultrasound contrast agents are present, then clotting happens even more rapidly.

Dr Lawrence Crum, one of the researchers, said: "It's a phenomenon we're really excited about."

However, a UK stroke expert said that the practical benefits of the techniques were not yet clear.

Professor Philip Bath, from the University of Nottingham, said that in many cases of bleeding on the brain, surgery would not be a recommended option.

He said that some brain injuries, such as those caused by accidents, might well benefit from the advance.

He said: "I'm not surprised that it might be sensitive to ultrasound. It's very interesting, but at this stage you would have to wonder how useful it would be."