The US has developed a pocket-size drone dubbed the Nano Hummingbird for the way it flaps its tiny robotic wings as a mini-spy plane capable of manoeuvring on the battlefield and in urban areas.

The battery-powered drone, which looks like a bird for potential use in spy missions, was built for the Pentagon's research arm as part of a series of experiments in nanotechnology. It is the result of a five-year effort, announced on Thursday by the Pentagon and AeroVironment of California.

Equipped with a camera, the drone can fly at up to 18km/h and hover and fly sideways, backwards and forwards, clockwise and counterclockwise, for about eight minutes.

Industry insiders see the technology eventually being capable of flying through open windows or sitting on power lines, capturing audio and video undetected.

The Hummingbird would be a departure from existing drones, which closely resemble traditional aircraft. The next step is likely to be further refinement of the technology, before decisions are made about whether the drones would be mass-produced and deployed.

''The miniaturisation of drones is where it really gets interesting,'' said Peter Singer, author of Wired for War, a book about robotic warfare. ''You can use these things anywhere, put them any place, and the target will never even know they're being watched.''

Critics have noted that privacy issues may emerge depending on how the drones are used.

With a wingspan of 16.5 centimetres, the mini-drone weighs 19 grams, or less than a AA battery. The Hummingbird's guts are made up of motors, communications systems and a video camera. It is slightly larger than the average hummingbird.

The Pentagon has awarded about $US4 million to AeroVironment since 2006 to develop the technology and the drone.