Blimps - large balloon-like contraptions high up in the air - have hitherto been used to advertise products. Now the US defence ministry has fitted blimps with sensors and cameras to spot enemy movements.

It has revealed that a 54 metre-long lighter-than-air craft will conduct test runs over Washington, where it first appeared on Wednesday. These tests would determine how effective the electro-optical and infrared cameras aboard are in detecting potentially threatening movements on the ground.

The advantage that blimps enjoy over other military hardware is that they are cheaper and can hover in the air for extended periods of time due to lesser fuel consumption.

Though blimps are out of range for small arms and have the option to guard itself against surface-to-air missiles but are vulnerable to bad weather, particularly high winds, said Navy Commander Mike Giauque, deputy project manager for the surveillance system.