Australia will buy at least a dozen armed Reaper drones in a major step towards a future in which air strikes are increasingly carried out remotely.

The aircraft, made by US company General Atomics, will be used mainly to provide cover to Australian soldiers on operations, flying overhead while looking for threats and striking them with bombs or missiles.

Defence Minister Christopher Pyne will announce the planned purchase of the new drones on Friday morning. Fairfax Media understands the Morrison government plans to buy between 12 and 16 of the aircraft which, at a cost of about $23 million each, will put a price tag of at least $400 million on the fleet when extras are included.

Mr Pyne, in a statement released before the announcement, said a Reaper could watch from the skies for longer than manned aircraft.