LOS ANGELES (AFP) - A growing number of wildfires were threatening people and property in the western United States on Tuesday (Aug 23), and the governor of Washington state declared a state of emergency in 20 counties.

"These fires threaten people, property and the natural resources of eastern Washington," Governor Jay Inslee said in a statement.

"This proclamation frees up state resources so we can provide the assistance that these communities might need. This is a time for all Washingtonians to come together."

The fires in the largely rural areas have destroyed more than 25 buildings and threatened hundreds more, triggering evacuations, Mr Inslee said.

They included the Cherry Road Fire, which has charred 12,800ha and is 50 per cent contained, while the Kahlotus Fire has burned 8,000ha and was 19 per cent contained, according to the Northwest Interagency Coordination Centre.

The 1,500ha Spokane Complex fire was threatening 167 residences, with two damaged and 12 destroyed.

Mr Inslee noted weather conditions in the next seven days will be hot and windy, hampering efforts by firefighters who are already short-handed.

Further south along the Pacific coast, 10,000 firefighters were battling six gigantic wildfires in California.

In some positive news, the Blue Cut Fire in San Bernardino County, just 100km east of Los Angeles, was contained on Tuesday morning.

It has forced the evacuation of more than 82,000 people, snarling traffic as highways across the region were closed.

The monster blaze has burned through 14,000ha, destroying 321 buildings including 105 homes. That makes it the 20th most destructive fire in California history, according to Cal Fire.

The Chimney Fire, which has burned 15,000ha in California's San Luis Obispo wine region, was raging just 3km east of the iconic Hearst Castle mansion, which remains shuttered, the Los Angeles Times said.

Also known as San Simeon, the castle once owned by newspaper baron William Randolph Hearst is one of the most recognisable landmarks in the state, drawing millions of visitors each year, according to local tourism officials.

The fire has already charred 52 buildings and threatened 1,900 others.

Across the country, 18,000 firefighters were fighting 32 huge fires, according to the National Interagency Fire Centre, mainly in western states.