Bluetooth may be more vulnerable than first thought after security consultants unveiled a device that can pick up transmissions up to 1km away.

Nicknamed the BlueSniper, the device consists of a directional 'yagi' antenna mounted on a foldable stock with a Bluetooth module and processor built into the magazine, although it can also be hooked up to a laptop.

"With multiple 'guns' it would be possible to track a single Bluetooth device as the person walked around," reported John Hering, of wireless security consultants Flexilis, who built the device.

"In less than a few minutes, 20 devices were detected all at distances over a half mile away. We decided to quickly conclude the scan, given police activity in the area earlier in the day from a bomb scare."

The BlueSniper is a development of an earlier Bluetooth hacking device which the company exhibited at the DefCon hackers convention last year.

The Bluetooth Special Interest Group, made up of manufacturers and developers, issued a advisory on Bluetooth security in January.

"We take security very seriously. There is a Security Expert Group in place to address new and existing vulnerabilities, and so far no security holes have been discovered in the Bluetooth Specification," the group said in a statement.

"Roadmap enhancements continue to address security concerns to keep Bluetooth technology as the secure wireless technology it is."