Cal Fire Engineer Clint Singleton monitors a hotspot from a hill during the Rocky Fire near Clearlake, California August 5, 2015. The fiercest of some two dozen large blazes currently raging across state is the so-called Rocky Fire, which has charred more than 68,000 acres since erupting July 29 in the foothills and canyons east of the town of Clearlake, about 110 miles north of San Francisco. [Photo/Agencies]

The tally of property losses from California's largest and most destructive wildfire this year nearly doubled on Wednesday as the week-old blaze raged through dry scrubland north of Napa Valley wine country.

The higher damage figures from the California Department of Forestry and Fire Prevention (Cal Fire) coincided with an ominous US Forest Service report that more than half its total budget is, for the first time, being spent on fire suppression across the country.

The agency's rising expenditures reflect an extraordinary wildfire season experienced this summer in California and several other Western states - including Oregon, Washington, Idaho and Alaska - in the midst of a widespread and prolonged drought.

Cal Fire crews have responded to more than 4,200 wildfires large and small so far this year, about 1,500 more than average.

The fiercest of some two dozen large blazes currently raging across state is the so-called Rocky Fire, which has charred nearly 70,000 acres since erupting July 29 in the foothills and canyons east of the town of Clearlake, about 110 miles north of San Francisco.

The fire has so far reduced 91 structures to ash - 39 homes and 52 outbuildings - up from the 50 structures previously counted as destroyed, Cal Fire reported on Wednesday. The agency warned that the toll of property losses may climb higher still as damage-assessment teams reach more recently burned areas.

On Wednesday, nearly 7,000 structures, mostly dwellings, remained listed as threatened, with some 13,000 people placed under evacuation orders or advised to leave their homes. Portions of two major state highways in the area are closed.

The containment level for the blaze, a measure of how much of the fire's perimeter has been corralled by natural barriers or within buffer lines carved by ground crews, rose to 30 percent, up from 20 percent the day before, even as the flames devoured another 2,500-plus acres of landscape, Cal Fire said.