ANGELS CAMP, Calif. -- Hundreds of people rushing to escape a massive wildfire charging across the tinder-dry Sierra Nevada foothills said Saturday that they had to make wrenching decisions about what to save -- pets, loved ones' ashes -- and what to leave to possibly burn.

A blood-red sun pushed through a choking fog of smoke and ash that turned the grassy, tree-studded area about 70 miles (113 kilometres) southeast of Sacramento an eerie white. Away from the burned-out cars and smouldering remains of homes, Annette Stout and other residents who fled the flames rested at evacuation centres.

Stout was ordered from her house Friday afternoon, and for the first time since her husband's death in March, she drove their recreational vehicle to safety in Angels Camp, a quaint town made famous by Mark Twain's "The Celebrated Tale of the Jumping Frog of Calaveras County."

"I grabbed my cats, their carriers, important papers, my husband death's certificate and his ashes," said Stout, who lives in the community of Hathaway Pines.

Despite the outpouring of help at the centre set up at the Calaveras County Fairgrounds, she didn't sleep well.

"We knew we were safe here, but (I was) worrying about the house, worrying about those who didn't leave," she said.

The blaze that ignited Wednesday exploded to more than 100 square miles (260 square kilometres) in two days amid temperatures above 100 degrees Fahrenheit (38 degrees Celsius) and land parched from several years of drought. But crews gained some ground Saturday, increasing containment slightly despite smoke grounding helicopters and air tankers, state fire officials said. The fire has destroyed at least 15 buildings and threatened some 6,400 more.

Michelle Griffiths checked on livestock after spending much of the night rescuing her neighbours' four horses and seven cats in the community of Mountain Ranch.

"People were running for their lives two nights ago," which is when her neighbours left their house and livestock for a motel, Griffiths said.

"Fortunately, our house is still standing" and so is the neighbours', she said.

Cooler weather was forecast for later Saturday, but people in nearby San Andreas, a gold-rush town of 2,700, have been told they may have to evacuate.

Gov. Jerry Brown declared a state of emergency for the blaze burning in both Calaveras and Amador counties, helping to free up funding and resources in the firefight.

There are 3,000 firefighters assigned to the blaze that ignited Wednesday, and more expected to arrive throughout the day. Its cause is under investigation.

Meanwhile, another California wildfire threatened to sweep through an ancient grove of Giant Sequoia trees. The lightning-caused fire has charred 172 square miles (445 square kilometres) and grew by nearly 40 square miles (104 square kilometres) in the last week.

In a fight to save the trees, firefighters have been clearing lines with bulldozers around the Grant Grove and putting up sprinklers. Firefighters continued to fortified containment lines Saturday, the U.S. Forest Service said.

The grove is named for the towering General Grant tree that stands 268 feet (82 metres) tall. There are dozens of Sequoia groves in the Sierra Nevada, and some trees are 3,000 years old.