Pushing in three directions, a massive wildfire in hills near Los Angeles doubled in size to 85, 000 acres (about 34,425 hectares) overnight, authorities said on Monday.

An air tanker makes a retardant drop at the west flank of the Station Fire near Los Angeles August 30, 2009. The wildfire burning out of control in the mountains of Los Angeles doubled in size overnight on August 31, 2009 and torched more structures on its march toward rural communities, officials said. [Xinhua/Reuters Photo]

"That fire burned just like it was daytime. Usually you get recovery because humidity goes up at night, which slows the fire down and you're able to construct more line around the fire," said U.S. Forest Service spokesman Nathan Judy. "But last night that wasn't the case."

The Station Fire, which has destroyed 21 homes and killed two firefighters, is threatening 12,500 homes, forcing thousands of residents to flee. While the fire spreads, more homes will be burned, officials said.

Authorities put 6,600 residents under mandatory evacuation orders Sunday night.

Firefighters prepared for another long, hot day, putting significant resources on the northern edge of the fire near the town of Acton.

"We are making progress, but it is very slow and very dangerous," incident commander Mike Dietrich of the U.S. Forest Service said at a news conference this morning. "We have to wait for the fire to come to us."

Two firefighters were killed when the car they were in ran off the side of a treacherous road in the Mt. Gleason area, south of Acton, Sunday afternoon, said LACFD Chief Mike Bryant.

"This accident is tragic," Bryant told a press conference Sunday evening. "This is a very difficult time for L.A. County Fire Department and the men and women that serve day in, day out."

The fire was only five-percent contained and full containment was not expected until Sept. 8, officials said.

"In this rugged, steep terrain, with this brush as thick as it is, we are having difficulties establishing containment lines where we can make a stand," said Capt. Mark Savage, a spokesman for the LACFD. "This fire is still very much out of control."

More than 2,800 fire personnel from around the state have converged to battle the Station Fire, along with 12 helicopters and eight air tankers.

The hills of Acton, California are dotted with flames at the Station Fire's northern front August 30, 2009. [Xinhua/Reuters Photo]

Meteorologists predicted that hot, dry conditions would continue without relent until at least Tuesday.

Meanwhile, the Southern California Coast Air Quality Management District (AQMD) reported that smoke and ash from the fire caused unhealthy to hazardous air quality in the San Gabriel Mountains and the west San Gabriel Valley.

"Very high concentrations of fine particulates are occurring in areas of direct smoke," according to an AQMD statement.

The AQMD advised residents in smoke-affected areas to avoid vigorous outdoor or indoor activity.

The fire, which broke out Wednesday afternoon, was the largest of many burning around California.

The cause of fire was still under investigation.