It's a bird! It's a plane! It's a ..... skysail?

European air safety and navigation agency, Eurocontrol, is warning pilots that a new form of collision threat exists from ships that have begun experimenting with large high-flying kites or sails for auxiliary propulsion.

They know this because they received a report about a 15 September incident in which an aircraft over the southern North Sea reported seeing a kite or sail flying at approximately 1,000ft height "just below the clouds" ahead of a ship as it headed into port.

The agency did not revealed the type or operation of aircraft involved, but did post an artist's representation of what this newfound (and logical) auxiliary propulsion device might look like.

"It was attached to the vessel and was in the flight path of the aircraft as it headed towards  a nearby installation," the agency reports in a safety warning message issued today. "The 'skysail' was extended on a long cable and was moving around the vessel in an erratic manner."

Officials later determined that the ship in question is "the first in a line of experimental vessels using skysails to supplement the traditional propulsion units".