* More than 70 dwellings confirmed destroyed
* Over 6,000 homes under evacuation orders
* Fire crews retreat from key mountain summit (Updates with new damage toll, evacuees, news conference)
LOS ANGELES, Aug 31 (Reuters) - A massive wildfire roaring through mountains north of Los Angeles forced some firefighters to retreat on Monday as the toll of engulfed homes rose sharply and flames menaced Mount Wilson, a broadcasting hub and site of a historic observatory.
Fire officials said at least 71 dwellings had been destroyed since the blaze erupted last Wednesday -- 53 clustered in the foothill community of La Crescenta on the northern fringe of suburban Los Angeles and 18 others first reported as lost on Sunday.
As of Monday evening, a total of 6,300 homes throughout the fire zone were under evacuation orders, authorities said.
The so-called Station Fire more than doubled in size as it burned out of control for a sixth day, charring 105,000 acres (42,500 hectares), up from 42,000 acres (17,000 hectares) late on Sunday, and sending up towering palls of smoke that fouled the air for miles (km) around.
Two firefighters died on Sunday when they were overrun by flames in the Angeles National Forest and rugged San Gabriel Mountains. Nearly 2,600 firefighting personnel, some from as far away as Montana and Wyoming, were battling the blaze.
With no forecast for an immediate break in the triple-digit temperatures and very low humidity that has helped energize the fire, officials pushed back their projected date for full containment of the blaze by one week to Sept. 15.
"This is a very angry fire that we're fighting right now," Fire Commander Mike Dietrich told a news conference. "Until we get a change in the weather, I'm not overly optimistic."
BATTLE FOR A MOUNTAINTOP
Fire crews fought to protect the slopes around the 5,700-foot (1,740-metre) peak of Mount Wilson, home to 50 buildings plus a famous array of telescopes and a critical cluster of transmission towers for broadcasters.
After dousing the area in fire retardant and laboring to clear brush away from structures on the site, they fell back early on Monday to avoid flames expected to sweep the summit.
"They've done everything they can do and it's unsafe for them to be there when the fire hits," Los Angeles County Fire Captain Mark Whaling said.
Elsewhere in the forest, 65 firefighters retreated from a wall of flames advancing on their positions, he said.
Rescue teams were standing by to save five people who became trapped in the forest after they disregarded evacuation orders, authorities said.
The fire threat eased in some foothill communities that were menaced over the weekend. Other neighborhoods were now at risk, including about 300 homes on the southwest flank of the blaze just inside Los Angeles city limits.
At the end of one cul de sac in the community of Tujunga, residents stood outside their homes, their cars and trucks packed with belongings and pointed out of their driveways, nervously eyeing smoke billowing from over the steep hillsides that flanked their neighborhood. Police in patrol cars cruised up and down the streets, urging residents to leave.
"If flames start coming down (the hills), we're out," said Jodi Befu, 49.
"Waiting is the hardest part," said David Jones, 44. "I'm all packed. I've had my wife and my baby in a hotel for the last two nights." He insisted everyone in the neighborhood would go if the flames drew much closer.
"Nobody here is going to be a hero," he said.
The first day of classes for two school districts were canceled due to heavy smoke, and the Los Angeles Department of Water and Power asked customers to curb electric use in case of potential fire damage to a pair of 500-kilovolt transmission lines northeast of the town of Acton.
The blaze was being fueled by dense, tinder-dry vegetation that had not burned in decades. So far, the Santa Ana winds that fanned many of Southern California's worst wildfires in recent years were absent.
Three civilians were reported injured over the weekend, including two who defied evacuation orders and sought shelter in a hot tub when the flames arrived. (Editing by Dan Whitcomb and Peter Cooney)
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