Rescue vehicles rush to put out wildfires near Arcadia, California, 20 kilometers away from Los Angeles on Sunday local time. Residents were ordered to leave due to the out-of-control fire. (Photo: Xinhua)

A major fire that has been raging outside Los Angeles for more than a week threatened to engulf a historic observatory and billion-dollar broadcast towers on Tuesday as firefighters struggled to contain the flames.

The so-called Bobcat Fire was within 500 feet (150 meters) from the 116-year-old Mount Wilson Observatory, the US Forest Service said in a tweet, while fire officials said crews were in place "ready to receive the fire."

Officials at the observatory said in a tweet late Monday that all personnel had been evacuated as the fire was "knocking on our door."

Firefighters battling the blaze had made slight headway in recent days in trying to control the flames that erupted September 6, but containment shrank from 6 percent to 3 percent Tuesday, according to the Angeles National Forest.

"They are in a firefight right now, because it is so close," LA County Fire Captain David Dantic told the Los Angeles Times, referring to crews positioned at Mount Wilson.

He said the fire, located about 16 miles (25 kilometers) northeast of downtown Los Angeles, had grown to more than 40,000 acres.

"It's a bigger area now," Dantic said. "Before, we had 6 percent containment when it was about 30,000 acres, but now the fire has gotten bigger. It's a bigger footprint. That's why the containment is down."

KNX radio said the fire was also threatening broadcast towers in the area worth more than a billion dollars.

Many communities threatened by the flames have been ordered to stand ready to evacuate while some 80 historic cabins located within Los Angles Forest were feared destroyed, officials said.