Drones have been seen flying over several sensitive landmarks in Paris in a night-time mystery that French police say is being taken seriously. At least five of the unmanned aerial machines were spotted between midnight on Monday and 0600 on Tuesday, and none of the operators has been caught. The first appeared over the US embassy and there were later sightings near the Eiffel Tower and Place de la Concorde. French police believe the drone flights could be linked. Small drones are easy to buy but their appearance in recent months over sensitive locations has worried French authorities.