Lake Fire California
A helicopter battles a Lake Fire blaze in the San Bernardino National Forest. Aircraft had to be called back because of a hobby drone in the area.KTLA5 via YouTube

A massive 21,000-acre (8,500 hectare) California wildfire just got worse because a mystery drone interfered with a water drop.

Officials called back aircraft preparing to dump water and fire retardant on the flames devouring a section of the San Bernardino National Forest in southern California because the drone was zipping through the airspace presenting a serious risk to pilots and firefighters, authorities told the Los Angeles Times.

After the delivery of water and fire retardant was halted an additional 3.5 square miles of forest burned in the Lake Fire, reports local KTLA-TV. More than 7,000 structures are now threatened in the area and mandatory evacuations are in effect around the fire.

"We don't want to deal with unknown aircraft in our airspace," Mike Eaton, forest aviation officer for the forest, told KTLA. "They've got enough on their mind already ... difficult terrain, difficult weather, winds and other things. They don't need to be worrying about model airplanes or drones."

The orange drone was illegally flying too high and in a restricted fire zone. "If you fly, we can't," Eaton said. "It's breaking the law."

The owner of the drone is not known.

The US Forest Service issued a statement on civilian drones last year, warning they could interfere with firefighters' efforts to battle wildfires in the air. Last year law enforcement confronted a drone operator in northern California after he flew his craft near a wildfire as crews made water drops. He told police he was using the drone to photograph the fire.