The fire, which began shortly before 6 p.m., was mostly burning in the Angeles National Forest, away from populated areas, and was 5 percent contained shortly before midnight.
Residents of three foothill homes closest to the blaze had been advised to evacuate, said Robert Brady, a fire information officer for the U.S. Forest Service.
Although the fire was burning away from that area and into the forest, those homes and others in Azusa could be threatened if the winds change direction and push it to the south, said U.S. Forest Service spokeswoman L'Tanga Watson.
About 20 homes are in the vicinity of the fire, officials said.
Brady said some 200 firefighters from the Forest Service and Los Angeles County were battling the flames in the mountains about 25 miles northeast of Los Angeles. The fire's cause wasn't immediately known.
The blaze sent a huge plume of gray smoke billowing into the air as so-called Super Scooper planes doused it with water they sucked up from the nearby San Gabriel Reservoir. Those aircraft were grounded as darkness fell, but smaller water-dropping helicopters continued to douse the flames into the night.
Although temperatures remained near 80 degrees late into the evening, Brady said weather conditions, overall, were good for firefighting. Winds were blowing at only about 5 mph, he said, helping slow the flames progress.
"At the beginning it was running pretty good, but it seems to have calmed down," he said late Monday night.