Flames have raced across treetops and through drought-stricken vegetation as firefighters scrambled to keep a growing California wildfire from reaching a resort city at the southern tip of Lake Tahoe.

They battled the blaze as evacuation orders were expanded to the neighbouring state of Nevada.

Thick smoke from the Caldor Fire enveloped the city of South Lake Tahoe, which was all but deserted during a summer week usually bustling with tourists.

The National Weather Service in the US warned that critical conditions through the day could include extremely low humidity and gusts of up to 30mph.

"With those winds, as it ran through the forest it created what's called an active crown fire run, where the fire actually goes from treetop to treetop," said Stephen Vollmer, a fire behaviour analyst for the California Department of Forestry and Fire Protection.

He said embers were being cast up to a mile out in front of the fire, creating new ignition points.

The blaze was three miles outside of South Lake Tahoe by Tuesday afternoon, fire officials said.

A day earlier, roughly 22,000 residents jammed the city's main artery for hours after they were ordered to leave as the fire advanced.

South Lake Tahoe city officials said only a handful of residents defied the evacuation order. But nearly everyone is worried about what the fire will do next.

Tom O'Connell and his wife, Linda, awaited the fate of their home while anchored on their boat in Ventura Harbour.

The two-bedroom they have owned for 40 years survived the Angora Fire that destroyed about 250 houses in 2007. They did not know if they would be lucky again.

"You worry about the things you can have some control over," Mr O'Connell said. "We've no control over this."

Pushed by strong winds, the Caldor Fire crossed two major roads and swept down slopes into the Tahoe Basin, where firefighters working in steep terrain were protecting remote cabins.

Thick smoke prevented air firefighting operations periodically last week, but since then nearly two dozen helicopters and three air tankers have dumped thousands of gallons of water and retardant on the blaze, said fire spokesman Dominic Polito.

The Lake Tahoe area is usually a year-round recreational paradise offering beaches, water sports, hiking, ski resorts and golfing. South Lake Tahoe bustles with outdoor activities while just across the state border in Stateline, Nevada, tourists can gamble at major casinos.

But on Tuesday, only a few dozen tourists remained on the casino floor of the Montbleu Resort, Casino and Spa. The state board that controls gaming said that casino regulators were monitoring operations at the four largest gambling properties in the city.

Hotels are housing evacuees, fire crews and other emergency personnel.

Nevada governor Steve Sisolak urged residents to be prepared, saying there was no timeline for when evacuations might be ordered. At a news conference in Carson City, he noted that ash was falling on him even though the fire was about 20 miles away.

"I'm standing here and I'm getting all ash particulates on my jacket, even," the governor said. "This is serious, folks."