Fire crews in California have fought back a huge, wind-driven wildfire, leaving the resort town of South Lake Tahoe unscathed as flames crept toward Nevada.

US President Joe Biden approved a declaration of emergency in California and ordered federal assistance to boost local responders' efforts to battle the Caldor fire, the White House said.

President Biden's action authorised coordination of disaster relief measures by the Department of Homeland Security and Federal Emergency Management Agency (FEMA).

The Caldor fire, burning since mid-August in the Sierra Nevada range, still threatened homes and businesses near Lake Tahoe, made worse by gusty, bone-dry conditions.

Forecasts suggest winds will significantly subside today and tomorrow, giving firefighters a chance to make more headway.

"Over the next couple of days, we're going to see the weather change, and we're going to see the fire behaviour slacken ... to the point where we can actually get in there and do some good work," Steve Volmer, a fire behaviour analyst with the California Department of Forestry and Fire Protection (CalFire), told a community briefing yesterday evening.

A firefighter watches as the Caldor Fire approaches Meyers, California

South Lake Tahoe, the biggest town along the rim of North America's largest alpine lake in a region renowned as an outdoor recreational hub and world-class ski area, had appeared to be in grave danger 24 hours earlier.

Its 22,000 residents were ordered to evacuate on Monday after flames unexpectedly swept down towards communities in the lake's basin.

Over the next two days, firefighters battled to save South Lake Tahoe and nearby communities, including the smaller, unincorporated village of Meyers, a former trading post and Pony Express station.

By yesterday morning, they had "steered the fire away," Jason Hunter, a spokesman for the Caldor incident command said.

Mr Hunter said the wind's direction late on Tuesday through yesterday morning had helped push flames to the northeast rather than straight north toward Lake Tahoe, which straddles the California-Nevada border.

"There was a massive amount of heavy-equipment work and structure-protection work along those neighborhoods," he said.

"We're not out of the woods yet, but crews kept the fire outside those communities and away from homes," Cal Fire spokesman Henry Herrera said.

Firefighters also got help yesterday morning from an atmospheric inversion layer that settled over the Tahoe area, trapping smoke close to the ground and tamping down flames at lower elevations, officials said.

The Caldor Fire approaches a community of homes in Meyers, California

In all, Caldor has forced an estimated 50,000 people to flee and destroyed at least 729 structures, most of them single-family dwellings, while 34,800 more buildings were listed as threatened throughout the fire zone, Cal Fire said.

As of yesterday morning, the fire had burned more than 204,000 acres (82,500 hectares) of drought-parched timber, some 4,700 acres (1,900 hectares) more than reported the night before.

Ground crews hacking away vegetation with bulldozers and hand tools had carved containment lines around 20% of the fire's perimeter as of yesterday, Cal Fire said, up from 15% containment on Tuesday morning.

The Caldor fire was 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.

While winds were expected to abate, extremely low humidity remained in the forecast.

As the fire raged closer to the Nevada state line, officials expanded the area under evacuation orders to include more of California's sparsely populated Alpine County, on the border.

Authorities across the state line in Douglas County, Nevada, warned residents to be ready to flee at a moment's notice.