Coming winds and warmer temperatures brought worries that progress made in slowing the growth of a massive wildfire in Northern California could be undone.

On the heels of cooler, wetter conditions around the King Fire east of Sacramento that has burned 137 square miles and destroyed 10 homes, Tuesday is expected to bring winds up to 30 mph and low humidity that could bring a new surge.

"This could set up some potential fire growth similar to what we experienced when it grew exponentially last week," said state fire spokesman Capt. Tom Piranio. "We are working very aggressively to maintain the contingency lines."

A red flag warning of very high fire danger has been issued Tuesday and Wednesday and could bring even greater gusts.

"It's definitely going to pose a challenge to the control lines the firefighters have created," said Holly Osbourne, a National Weather Service meteorologist in Sacramento. She said Thursday will bring a slight chance for rain.

On Monday, more than 5,000 firefighters — from as far as Florida and Alaska — scrambled to extend control lines in expectation of tougher conditions. The blaze was just 18 percent contained nearly 10 days after it started.

It threatens about 21,000 structures, more than half of them homes. It has destroyed 10 houses and 22 minor buildings in the White Meadows area of Pollock Pines.

About 2,700 people remain under evacuation orders, said state fire spokesman Daniel Berlant. The fire also continued to threaten a key University of California, Berkeley, research station that is home to scores of experiments on trees, plants and other wildlife.

The blaze was spreading smoke beyond California, with the weather service issuing a "dense smoke advisory" about 100 miles away in Reno, Nevada, and Lake Tahoe on Monday. Shifting winds later pushed the worst of the smoke back west.

Those winds sent the air quality index into the "very unhealthy" level for general populations in Auburn northeast of Sacramento — where some schools closed, as well as the Roseville and Rockville areas.

A man charged with starting the fire, Wayne Allen Huntsman, 37, pleaded not guilty to arson Friday. He remains in the El Dorado County jail on $10 million bail.