LOS ANGELES (Reuters) - A wildfire burning out of control in mountains and foothills east of Los Angeles mushroomed more than 50 percent overnight, forcing authorities to order three school districts to cancel classes due to heavy smoke and dangerous conditions.

More than 900 firefighters were battling the so-called Pilot Fire, which has charred some 7,500 acres of bone-dry tinder and brush in the San Bernardino Mountains since it broke out around noon on Sunday.

“We feel it is in the best interest of safety that we keep our students and staff at home,” the Silver Valley Unified School District, which oversees nine schools in Mojave Desert, said in a statement on its website.

Also closing campuses on Tuesday were the Apple Valley and Hesperia school districts in those high desert communities some 90 miles east of Los Angeles.

More than 5,000 homes were under evacuation orders from the Pilot Fire, a highway and several roads were closed and smoke advisories were issued for the Mojave Desert area, according to the California Department of Forestry and Fire Protection (Cal Fire).

No homes had been destroyed but Cal Fire said the blaze was only 6 percent contained as of Tuesday morning.

Some 400 miles to the north, the famed Highway 1 along the California coast was reopened to residents, one day after authorities were forced to close it in both directions due to the threat from the Soberanes Fire.

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That blaze, which erupted on July 22, has already blackened 67,000 acres in the Big Sur area, destroying 57 homes and 11 outbuildings.

A bulldozer operator died on July 26 when his tractor rolled over as he helped property owners battle the flames, the sixth wildfire fatality in California this year.

Authorities have traced origins of the blaze to an illegal campfire left unattended in a state park about a mile from Highway 1. No arrests have been made so far.

As of Monday, more than 4,800 firefighters battling the flames had cut containment lines around 50 percent of its perimeter.

Firefighters are making gradual progress against the blaze as wildfire season in the western United States reaches its traditional peak, intensified by prolonged drought and extreme summer heat across California.

The conflagration is one of 35 major wildfires that have charred half a million acres in 12 states, mostly in the West, according to the National Interagency Fire Center in Boise, Idaho.