Thousands of people remain under threat of evacuation as firefighters battle wildfires in the U.S. state of Oregon. Blazes are raging in other Western states from California to Colorado.

Two fires that have charred more than 70,000 hectares in southern Oregon continue to threaten several towns near the California border. Officials say the fires have doubled in size since Tuesday.

In central California, a fire that has burned more than 33,000 hectares of the Sequoia National Forest has so far spared the groves of historic redwood trees.

Farther south, a smaller fire has destroyed at least four homes near the mountain resort of Julian, northeast of San Diego.

Tracy Gouette of the U.S. Forest Service explains that the blaze is fueled by dry underbrush and dead foliage. "They've got conifers, firs, a number of different types of trees," she said. "They got a beetle that has infested some of those trees and killed them. When they're dead, they ignite very, very quickly."

Firefighters in southwestern Colorado are trying to protect archeological treasures at the Mesa Verde National Park, which was once home to the cliff-dwelling Pueblo Indians. The blaze has forced the park's evacuation.

Marty O'Toole of the National Interagency Fire Center points out this is one of the worst fire seasons on record in the Western United States. "Our forests are in a fairly dangerous condition," he said. "Fire suppression in the last 50 to 75 years has allowed the fuels, the wood and the leaves and pine needles, to build up and they have built up in some cases to critical levels."

That buildup, and a multi-year drought and hot summer season, have left some trees more dry than the timber sold in a lumberyards, according the fire center official.

U.S. wildfires have burned more than 1.5 million hectares this year, more than double the average level. The fires have also cost the lives of 20 firefighters. The latest victim was an helicopter pilot who died in a crash in Colorado Tuesday.