Firefighters battling California’s Lake Fire in the San Bernardino Mountains have battled strong winds, low humidity and fatigue as they struggle to contain a blaze that has consumed more than 20,000 acres since it began last week.

Now, firefighters have to contend with a new threat — drones piloted by careless hobbyists.

According to the Associated Press, several air tankers dropping fire retardant on the blaze Wednesday evening were forced to land after one of the pilots spotted a drone flying in the area.

Daniel Berlant of the California Department of Forestry and Fire Protection confirmed that five air tankers were ultimately forced to land due to the drone sighting.

Fire officials could not find the drone operator. Efforts to battle the blaze from the air resumed Thursday morning.

San Bernardino fire officials said the blaze was just 21 percent contained as of early Thursday morning. That containment figure is down significantly from 38 percent containment on Wednesday.

Firefighters were also working Thursday to extinguish the massive Washington Fire in Alpine County near Markleeville. That fire had grown to 17,205 acres by Thursday and was just 10 percent contained.