London - Drones are being flown at up to 600m above the ground, far above the height limit, and pose a growing risk to aircraft, Britain's civil aviation regulator warned on Wednesday."Interest in drones has developed rapidly in the last couple of years and our main concern is to ensure owners of drones can enjoy this rapidly growing technology safely and have regard for all other airspace users when doing so," said Tim Johnson, director of policy for the Civil Aviation Authority."Drone users must understand that when taking to the skies they are entering one of the busiest areas of airspace in the world," Johnson said.He said the CAA runs "a complex system that brings together all manner of aircraft including passenger aeroplanes, military jets, helicopters, gliders, light aircraft and now drones".It launched a drone safety awareness day on Wednesday following "a number of recent incidents involving drones and various aircraft."In each incident drone users flew their devices well above the height limit for drones, sometimes as high as 600m and in flight areas used by large aircraft, the regulator said.Drone users are required to keep them within their "visual line of sight", meaning the maximum height should be about 125m. The drones should remain at least 50m away from people and buildings, it said.The warning follows two near misses in recent months at London's Heathrow Airport, one the world's busiest aviation hubs.In December, a pilot reported seeing a helicopter-style drone about 6m from a wing of his an Airbus A320 passenger plane as he approached Heathrow at about 213m. The CAA said the incident posed a "serious risk of collision". Another drone passed 15m above an Airbus A320 as it approached Heathrow in March, according to an air safety report.