(Reuters) - A lightning-sparked wildfire roared unchecked on Sunday through tinder-dry grass, brush and timber in north-central California, where an estimated 3,000 people were forced from their homes in several small, rural communities, authorities said.

The blaze, which erupted on Saturday amid some 900 lightning strikes unleashed by thunderstorms that rolled through Northern California, has scorched at least 12,000 acres of private and public land along the border of Shasta and Tehama counties, officials said.

There were no reports of injuries, but the so-called Ponderosa blaze has destroyed at least seven homes and was threatening about 3,000 more, said Mary Anne Aldrich, a spokeswoman for the California Department of Forestry and Fire Protection.

She said about 2,000 homes in several communities, including the towns of Manton and Shingletown, about 125 miles north of Sacramento, the state capital, were ordered evacuated starting on Saturday night.

A rough estimate of 3,000 people were believed to have displaced by the evacuation, but precise numbers were hard to come by, Aldrich said.

So far, a force of nearly 1,000 firefighters, 20 water tenders and three helicopters had yet to curtail the fire's growth, officials said.

Hundreds of miles to the northeast in Idaho, a separate wildfire raged for a ninth day through rugged, sun-baked terrain in the Boise National Forest, where firefighters kept up a battle to prevent flames from reaching a newly evacuated town east of the state capital.

The Trinity Ridge blaze has swallowed well over 88,000 acres of sagebrush and timber in south-central Idaho since it was ignited on August 3 by an off-road vehicle that caught fire, authorities said.

The Idaho and Northern California fires were among nearly three dozen large conflagrations burning out of control on Sunday through nine drought-stricken Western states, devouring well over 1 million acres, according to the National Interagency Fire Center in Boise.

Except for a U.S. Forest Service firefighter killed by a falling tree last Sunday in northern Idaho, no major injuries or loss of life have been reported during the past week.

Red-flag warnings for high fire hazards were posted through much of the West on Sunday as forecasts called for possible thunderstorms that could bring more lightning and gusty winds.

Outside of California, the Trinity Ridge Fire continued to pose one of the greatest imminent threats to property in the region.

As of Sunday, firefighters had managed to carve containment lines around just 5 percent of the fire's perimeter while keeping the blaze from overrunning the former mining town of Featherville on the banks of the South Fork Boise River.

The tiny village, located about 50 miles east of Boise, is home to fewer than 100 year-round residents, but vacation homes and rental cabins swell its summertime population to as many as 1,000.

The town was evacuated on Saturday due to thick smoke and reduced visibility from the fire, and the only road leading to Featherville was closed to all but emergency traffic. It was not clear how many people were still in the area when it was ordered cleared, but fire information officer Mallory Eils said nearly 30 people were known to be staying put in town.

At least one leading edge of the sprawling fire was reported to have crept to within 3 miles of town.

Although smoke from the fire has created unhealthy air that is especially hazardous to children, the elderly and people with respiratory ailments, it has paradoxically helped slow the fire's advance at times by blocking out the sun, cooling temperatures on the ground.

On Sunday, crews stood by to protect buildings in town as firefighters prepared to torch swaths of land on its outskirts to remove brush, trees and other vegetation that could otherwise act as fuel for advancing flames, authorities said.

(Writing and additional reporting by Steve Gorman; Editing by Stacey Joyce)