Those hoping to find a drone under their Christmas tree, take note: Starting Monday, federal registration is required.

The new rule lands amid growing concern over the soaring popularity of camera-equipped hobby drones — and a sharp rise in sightings by alarmed commercial pilots.

As of Dec. 6, there were 1,158 unmanned aircraft sightings reported by pilots or crew in the United States, including some close encounters, according to the Federal Aviation Administration. That’s up from 238 such sightings in 2014.

“We expect hundreds of thousands of model unmanned aircraft will be purchased this holiday season,” said FAA Administrator Michael Huerta. “Registration gives us the opportunity to educate these new airspace users before they fly so they know the airspace rules and understand they are accountable to the public for flying responsibly.”

But skeptics think the move will do little to clear the skies of dangerous drones.

Responsible users will register; reckless ones won’t, predicts Richard Green, president of Babylon R/C Flyers, a hobby aircraft club chartered with the Academy of Model Aeronautics.

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“The guy that’s flying near an airport or near a plane — he’s not going to register,” Green said. “And there’s not going to be any enforcement as far as we can see because the FAA does not have that many inspectors out there to cover this.”

The rule covers drones and remote-controlled model airplanes that weigh between 0.55 and 55 pounds, and does not apply to commercially owned devices. The three-year online registration costs $5, refundable until Jan. 20, and requires owners to enter their name, email and home address, and a credit-card number, FAA officials said.

Owners must mark each drone with the federal registration number, and are required to keep their certificate handy when out flying. The number can be written in permanent marker or printed on a label, as long as it’s visible, officials said.

The owner of an unregistered drone could face civil fines up to $27,500, the FAA said. Criminal penalties include fines up to $250,000 and three years in prison. The deadline to register is Feb. 19.

FAA rules currently prohibit unmanned aircraft from flying within five miles of airports or higher than 400 feet, but reports of such incidents abound.

According to a recent analysis of 921 midair drone sightings by the Center for the Study of the Drone at upstate Bard College, 35.5 percent qualified as close encounters with manned aircraft. More than 90 percent occurred above 400 feet. The report also found the airspace around Kennedy, LaGuardia and Newark Liberty International airports had the most sightings, at 86.

The registration rule will help raise awareness of those dangers, said Phil Derner, founder of NYCaviation.com, an aviation news and consulting organization.

“It’s an important step to making people understand the responsibility of having it, the same way one would register a car or a gun,” he said. “These are things that are serious machines that come with dangers.”

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Derner expects drone enthusiasts to go through a learning curve on how to operate safely — and lawfully.

“Some people may not register right away, and it’s going to take making an example of someone,” he said. “Unfortunately, that’s probably going to happen when someone has done something wrong and possibly hurt someone in the process.”