Reports began surfacing last year of runaway drones interfering with air traffic and crashing into buildings. But the problem has grown worse as drone sales have surged.
"I'm definitely getting much more concerned about it," Federal Aviation Administration head Michael Huerta said. The FAA was particularly worried about a surge in reports of drones flying dangerously close to airports. The latest incident came on Monday, when four airline crews reported a brush with a drone on a flight path into Newark International Airport.
Huerta added that the recent interference by drones with California firefighters was "really a wake-up call for a lot of people. This kind of thing has got to stop".
Most new drone models are aimed at novice fliers who are often "blissfully unaware" of aviation safety practices, said Michael Braasch, an electrical engineering professor and drone expert at Ohio University.
"Unfortunately, there's also going to be a small percentage of users who are just going to behave badly."
The Consumer Electronics Association, an industry group, estimates that hobbyists would buy 700,000 of the remote-controlled aircraft in the US this year, a 63 per cent increase from last year.
Although the vast majority of drone enthusiasts fly solely for recreation, authorities worry about the potential for a new airborne menace.
In a July 31 intelligence bulletin, the Department of Homeland Security said it had recorded more than 500 incidents since 2012 in which rogue drones hovered over "sensitive sites and critical installations", such as military bases and nuclear plants. In one well-publicised case in January, a drone crashed on to the White House grounds.
Another unnerving scenario emerged last month when a Connecticut man posted an internet video of a drone he had armed with a handgun, firing shots by remote control as it hovered in the air. Local police and the FAA determined that no laws had been broken.
In general, drone misadventures are happening in a regulatory vacuum. The FAA has banned most commercial drone flights until it can finalise new safety rules - a step that will take at least another year.
But people who fly drones for fun are not regulated. Under a law passed in 2012 that was designed in part to protect model plane enthusiasts, the FAA cannot impose new restrictions on recreational drone owners. As a result, they are not required to obtain licenses, register their aircraft or undergo training.
To protect regular air traffic, the FAA has issued guidelines requiring that consumer drones stay at least 8km away from airports and below an altitude of 122m.
Those standards are widely flouted, however - in the past month alone, airline pilots have reported close calls with drones near airports in New York, Charlotte, Minneapolis and Phoenix.
In neighbourhoods nationwide, the buzz of drones is becoming a common sound, as well as a source of conflict. Police blotters contain an increasing number of reports from residents complaining about uninvited drones hovering over their back yards.
For the most part, such flights are legal - a fact that is prompting a backlash from anti-drone vigilantes.
In Hillview, Kentucky, last month, a homeowner blasted a drone out of the sky with a shotgun, saying he was trying to protect his daughters from being spied on. He was charged with criminal mischief - police did not take action against the drone owner.
Similarly, in May, a judge ordered a man from Modesto, California, to pay a neighbour US$850 ($1307) for peppering his drone with buckshot. In September, a man from Cape May, New Jersey, was charged with shooting down a neighbour's drone as it filmed houses along Seashore Rd.
In other cases, however, authorities have been more sympathetic toward drone haters. In June, prosecutors did not take action against a crew of firefighters in Orange County, New York, who used their water hoses to knock down a drone that had been filming them as they battled a house blaze.
In California last month, state legislators introduced a bill that would grant immunity to emergency responders who damaged a drone that got in their way. The measure was prompted by several incidents in which amateur paparazzi drones swarmed around wildfires, crowding the skies and forcing firefighters to ground their tanker aircraft to avoid a midair collision.
"Cars were torched on the freeways because drones made aerial firefighting efforts impossible," state Senator Ted Gaines said. "This is maddening and I can't believe that hobby drones are risking people's lives to get videos on YouTube."
Although the FAA lacks the authority to license recreational drones, it does have the power to impose civil fines on anyone who recklessly interferes with air traffic or endangers people on the ground. Huerta said yesterday that the recent spate of risky incidents prompted the agency to revisit its approach and that it would adopt "more stringent enforcement" measures in co-operation with state and local officials.
Brian Wynne, the president of the Association for Unmanned Vehicle Systems International, an industry group, said "there's always room" for drone companies to expand education efforts.
But he said there was only so much the industry could do to prevent reckless behaviour.
"I frankly just don't think there's any excuse for anyone flying a [drone] anywhere near an airport or near a runway," he said. "We have got to enforce our laws."
