Firefighters continue to wage an all-out battle to protect homes on the fringe of tinder-dry forests near Lake Tahoe from a wildfire that has chased thousands of people from the popular resort destination in California's Sierra Nevada range.

The Caldor fire, burning since mid-August in the mountains east of the state capital, Sacramento, crested a ridge line and roared downslope on Monday toward the southern end of the Tahoe basin that straddles the California-Nevada border, triggering mass evacuations.

Red Cross officials who have turned a Carson City recreation centre into a shelter are trying to make evacuees from the wildfire raging near Lake Tahoe as comfortable as possible.

Cots are placed at Covid-safe distances from each other on the gym floor, each topped with a crisp white blanket. Urns of coffee wait at the entrance.

But it is the information that is most craved by those staying at the centre in western Nevada and others who have found their own shelter after around 50,000 people were given evacuation orders because of the Caldor fire across the state line in California.

Forest Service staff stood at the door of the centre, pointing to a poster-sized map on an easel as they answered questions from evacuees about where the fire was in relation to people's homes.

A firefighter attempts to extinguish a spot fire off Highway 50 during the Caldor Fire in Meyers

Cal Fire spokesman Henry Herrera said that two large spot fires crept to within 5km of the southern boundary of South Lake Tahoe, a town with a population of 22,000 about 48km southwest of Carson City.

Officials said the fire is close to the unincorporated community of Meyers, a former trading post and Pony Express station almost 10km from South Lake Tahoe that has one grocery store, a hardware shop, a lumber yard and a handful of restaurants and shops.

Meyers residents are among those under evacuation orders.

"This thing is just unstoppable," Lee England, who fled her South Lake Tahoe apartment late on Sunday, said as she sat outside the Carson City centre yesterday evening.

Officials said the fire is close to the unincorporated community of Meyers

Before leaving, the 47-year-old initially thought - or hoped - she was seeing a storm in the distance.

"It was only wishful thinking that it was rain," she said. "It was smoke."

The smell of smoke hung in the air in Carson City. When ash fell on the back of Ms England's Boston terrier Bon Bon, she bent and gently plucked the gray fleck from the dog’s black and white fur.

A mile marker pointing in the direction of Placerville is surrounded by flames in Twin Bridges, California

As of yesterday, nearly 4,000 personnel and a squadron of over two dozen water-dropping helicopters were assigned to the blaze, whose cause remained under investigation.

"There is a substantial amount of resources right now dedicated to protecting the homes and property in South Lake Tahoe," US Forest Service spokeswoman Dana Walsh said.

Flames rise as firefighters try to extinguish the Caldor Fire in Meyers, California

A National Weather Service red-flag warning for dangerously gusty winds and extremely low humidity was posted for the Tahoe area through Wednesday night. But Ms Walsh said officials did not expect the wind to push the fire toward South Lake Tahoe.

Caldor has been burning since mid-August in the mountains east of Sacramento.

While it has not reached population centres such as South Lake Tahoe, at least 669 structures were listed as destroyed, most of them single-family homes, with 34,000 more buildings considered threatened.

By yesterday, the fire had charred more than 77,300 hectares of forest, some 5,665 hectares more than the day before. Firefighters had managed to carve containment lines around just 16% of its perimeter.

Caldor has been burning since mid-August in the mountains east of Sacramento

No deaths have been reported. Three firefighters and two civilians were injured in recent days.

Only the Dixie fire, which has charred 312,000 hectares further north in the Sierra, has burned more territory this year than Caldor.

Both fires are among nearly two dozen raging across California and scores of others elsewhere in the West, during a summer fire season shaping up as one of the most destructive on record.

The blazes have been stoked by extremely hot, dry conditions that experts say are symptomatic of climate change.