Helicopters and water-dropping tankers were back in the air to complement ground crews as more than 580 firefighters attacked the blaze. Calm winds and rising humidity also were expected to help the cause, said U.S. Forest Service spokesman Ed Gililland.
Acreage burned by the fire had reached 490 as of 6 a.m. Monday, authorities said. The containment percentage had dropped overnight from 30 to 23.
That was due in part to a fire surge in the early morning hours that pushed the past some containment lines and within yards of several houses.
But firefighters watered down embers that drifted onto a few roofs and fought back the blaze.
"This is pretty serious," said city of Sierra Madre spokeswoman Elisa Weaver. "Some of these areas have not burned in over 40 years."
Authoroties hoped to have the blaze fully contained within 4-7 days.
On Sunday, helicopters made water drops on a steep ridge above Sierra Madre near Bailey Canyon Wilderness Park, about 15 miles northeast of Los Angeles and just east of Pasadena. A fixed-wing water tanker also dropped flame retardant.
Aircraft also were helping fire officials assess the movement of the blaze, which had been creeping northwest into Angeles National Forest, Battalion Chief Tim Davis of the Forest Service said.
"It's very steep, inaccessible terrain, and it's very heavy brush," Davis said at a news conference. "Very difficult and arduous labor for these crews. You can't get bulldozers into the majority of where these fingers of fire run."
Two firefighters had minor injuries -- one was treated for heat exhaustion, another for a strained knee, authorities said.
The blaze stranded 50 guests from a wedding party at the Chantry Flats ranger's station on Saturday until they were airlifted out Sunday afternoon, Weaver said. It took five helicopter trips from the ranger's station to the parking area where the wedding party's cars were. The party then was escorted out by road.
Investigators were working to determine the cause of the fire.
To the south in San Diego County, about 100 acres burned in thick brush about 15 miles north of downtown San Diego.
The fire was 90 percent