The National Weather Service said the heat wave that brought temperatures of 54.4C to California's Death Valley on Sunday had likely peaked, with more moderate temperatures expected by the end of the week.

"However, excessive heat warnings remain in effect from California to Utah since readings will still be well above normal for Monday," the weather service said in an advisory, predicting triple-digit temperatures again in the desert southwest.

The unseasonable heat wave, triggered by a lingering high pressure system, is already the third for the region this year, an anomaly that some experts have attributed to climate change.

The high temperatures come as forests and brushwood are already bone dry across the West from years of severe drought, contributing to what authorities say could be an intense fire season.

Already in Oregon, the so-called Bootleg Fire had burned through more than 62,000 hectares in and around the Fremont-Winema National Forest. Crews have made little progress containing the blaze.

The flames were burning along a high voltage power corridor connecting Oregon's power grid with California's, worrying officials in both states that electricity could be knocked out to thousands of homes and businesses.

Hundreds of residents in the Klamath Falls area, in south-central Oregon, were under mandatory evacuation orders, and in an unusual step, county officials said residents who defy them could face citations or arrest.

Along California's border with Nevada, the Beckwourth Complex Fire had grown to 36,000 hectares on Monday morning, with about 23 per cent containment, according to the state's fire incident reporting system.

In Arizona, federal authorities on Monday were investigating the crash of a small plane that went down while surveying a wildfire in the northwestern part of the state, killing both crew members.

The fire had burned 300 hectares and was 75 per cent contained as of Monday morning, fire officials said.

Australian Associated Press