SAN DIEGO — As Jason Thrasher lowered his helicopter to a park with seven firefighters aboard, he saw what he thought was another firefighting chopper battling a blaze that was threatening homes.

The California Department of Forestry and Fire Protection pilot suddenly identified the object as a four-rotor drone only 10 feet from his windshield, forcing him to make a hard left to avoid a collision about 500 feet above ground, according to a report he filed the next day.

“If that drone came through my windshield, I have no idea what could have happened,” Thrasher said in a phone interview. “If that drone hits my tail rotor, for sure it’s going to be catastrophic.”

The near-miss last September in Nevada City, about 60 miles northeast of Sacramento, explains why drones have quickly become a serious nuisance and concern for firefighting pilots and other first responders, fueling calls for more oversight and self-policing in the skies.

The U.S. Forest Service has tallied 13 wildfires in which suspected drones interfered with firefighting aircraft this year — 11 since late June — up from four fires last year and only scattered incidents before. Last month, the sighting of five drones in a wildfire that closed Interstate 15 in Southern California and destroyed numerous vehicles grounded crews for 20 minutes as flames spread.