Firefighters struggled Thursday to contain infernos across the western United States as experts warned that drought-striken California should prepare for an unusually intense wildfire season.

Forest fires are a fact of life in much of California but have become far worse because of bone-dry conditions, with the Golden State gripped in its fifth year of drought.

A fire in the Los Padres National Forest had expanded to two square miles (five square kilometers) by Thursday, making it the "largest since 2009" in the area, a spokesman for the Santa Barbara County Information Center told AFP.

Strong winds were hampering efforts to contain the blaze, and the operation was expected to be hindered further by near-record temperatures over the weekend in the southern half of California.

Los Padres, which begins about two hours' drive northwest of downtown Los Angeles, is popular with hikers and campers, and evacuation orders were issued in at-risk parts of the forest.

Sections of Highway 101, which links northern and southern California, were temporarily closed while oil giant ExxonMobil evacuated its refinery in Las Flores Canyon.

Another fire further north burned about four square miles and caused road closures, also threatening buildings, although there were no reports of injuries.

Lynne Tolmachoff, a spokeswoman for public information organization Calfire, said America's most populous state could see its worst fire season on record this year.

Meanwhile, a blaze in Warren Creek, in the northwestern state of Alaska, was raging across eight square miles of a Native American reserve, while four fires were burning up more than 40 square miles in Arizona and New Mexico.

Last month fires near Los Angeles pushed 5,000 people out of their homes in the affluent Calabasas area, a suburb which is home to many celebrities including members of the Kardashian family.

The National Interagency Fire Center (NIFC) said the southwestern United States could expect "above normal levels of significant fire potential" through at least early July.

"The highest potential will be over Southern California during the first part of the summer as the past rainy season only brought 50 to 70 percent of normal rainfall," it said on its website.

"As the summer progresses, above normal significant fire potential area will expand northward to include much of the Sierras and the central coast region."

Wildfires in the western United States made 2015 the country's most devastating year since at least 1960.

More than 11 million acres (4.5 million hectares) -- an area greater than the size of Denmark -- had been burned by the end of summer, according to data from the NIFC.

The agency's data revealed a rise in "mega-fires" since 2000, attributed to factors including a growing number of homes in or near major forests and a trend toward hotter, drier seasons.

Last September, more than 1,500 homes were destroyed in two immense fires about 100 miles from San Francisco that caused millions of dollars in damage.