WASHINGTON — Pilot reports of drone sightings so far this year are more than double last year, government regulators say, raising concern about the potential for a deadly collision.

There have been more than 650 reports this year by pilots of drones flying near manned aircraft, the Federal Aviation Administration said in a statement Thursday. There were 238 unmanned aircraft sightings in all of 2014.

The reports come from pilots of a variety of aircraft, including many large airliners. The concern is that if a drone collides with an aircraft engine, it could disable the engine much like birds do sometimes when they get chewed up by engines. Also, a high speed collision with a drone might damage the surface of a plane, changing the airflows in a way that makes the plane difficult to maneuver.

There were 16 drone sightings reported in June 2014, and 36 the following month. This year, there were 138 reports from pilots flying up to 10,000 feet in altitude in June, and 137 reports in July.

Earlier this week, crews on four commercial flights spotted a drone while preparing to land at Newark Liberty International Airport in New Jersey. None of the pilots were required to make evasive maneuvers as a result of the sightings. The planes were between 2,000 and 3,000 feet in the air and eight to 13 miles away from the airport.

The FAA generally restricts drone flights to beneath 400 feet and at least 3 miles from an airport.

Flying a drone “anywhere near” an airplane can bring criminal charges and fines up to $25,000, according to the FAA.

Firefighters battling wildfires in the West have been forced to ground their operations on several occasions after drones were spotted.

Suspected drones interfered with aircraft fighting at least 13 wildfires so far this year, up from four fires last year and only scattered incidents before, according to the U.S. Forest Service.