The US military is developing miniature electronic sensors disguised as rocks that can be dropped from an aircraft and used to help detect the sound of approaching enemy combatants.

The devices, which would be no larger than a golf ball, could be ready for use in about 18 months. They use tiny silicon chips and radio frequency identification (RFID) technology that is so sensitive that it can detect the sound of a human footfall at 20ft to 30ft. The project is being carried out by scientists at North Dakota State University, which has licensed nano-technology processes from Alien Technology, a California-based commercial manufacturer of RFID tags for supermarkets.

It is an example of the increasing desire for the US military to co-operate with civilian industry and academic institutions in the development of battlefield technology that will reduce the risks to soldiers' lives. Greg McCarthy, associate vice president at the university's Center for Nanoscale Science and Engineering, said: "The military wants better sensing capability. People are being killed because someone's sneaking up on a tent and blowing them up."

“We've got some very realistic looking artificial rocks and stuck sensors in them,” he said. The military is already working with Detroit carmakers on the development of counter-terrorism and kidnap rescue vehicles. The new sensors would be made cheaply enough to be left on the ground without need for retrieval by soldiers. RFID technology uses radio signals that are sent from a silicon chip to a remote sensing device.

Phil Boudjouk, vice president of research and “creative activities” at NDSU said that before September 11, the NDSU and the Department of Defense had been working on using RFID to help track army inventories of items such as soldiers' boots and uniforms.

By January this year, all suppliers to the US military were required to put RFID tags on most cases and pallets delivered to the military. But the project's scope widened after the attacks. “9/11 changed the field of play dramatically,” he said. “We've had to enlarge our mission.”