There is an unconfirmed report that the US Marshal service has equipped aircraft with miniature mobile phone cells, nicknamed dirtboxes, that can locate mobile phones from the sky.

The G-men, who work for the courts and track down fugitives, supposedly have a small fleet of aircraft produced by Boeing called Digital Receiver Technology ( ergo the nickname DIRT ) that are equipped with the surveillance gear.

Comparing the uniquely identifying IMEI numbers stored in every mobile device, the onboard tech can be programmed to look out for specific handset.

The aircraft can triangulate the location of a handset once it detects it.

Boeing said: "DRT has developed a device that emulates a cellular base station to attract cell phones for a registration process even when they are not in use."

The US Department of Justice said: "What is done on US soil is completely legal. Whether it should be done is a separate question."