Hundreds of firefighters battled two large wildfires on Friday that threatened to merge near the most populated part of Oregon as the governor said dozens of people are missing in other parts of the state.

The state’s emergency management director Andrew Phelps said officials are “preparing for a mass fatality event” and that thousands of structures have been destroyed.

Governor Kate Brown said more than 40,000 Oregonians have been evacuated and about 500,000 are in different levels of evacuation zones, either having been told to leave or to prepare to do so.

Dozens of people are missing in Jackson County in the south and Marion County, where a fire continues to burn east of Salem, Ms Brown told a news conference on Friday.

Authorities also announced that a man had been arrested on two counts of arson for allegedly starting a fire in southern Oregon on Tuesday.

The Oregon Convention Centre in Portland was among the buildings being transformed into shelters for evacuees.

National Guard troops and corrections officers transferred about 1,300 inmates from a women’s prison in a southern suburb of Portland “out of an abundance of caution,” the Oregon Department of Corrections said.

A change in the weather, with winds dropping and shifting direction and humidity rising, greatly helped firefighters struggling to prevent the two fires from advancing farther west into more-populated areas.

“The wind laid down quite a bit for us yesterday. There also wasn’t that strong eastern wind that was pushing the fire more to the west,” said Stefan Myers of the state’s fire information team.

Winds coming from the Pacific Ocean also neutralised the fires’ advance and even pushed them back, Mr Myers said.

Almost 500 personnel were working on the fires, which were just a few miles apart, with rugged terrain between them that limits boots-on-the-ground efforts to keep them apart, Mr Myers added.

If they merge, they could generate such heat that it causes embers to fly thousands of feet into the air, potentially igniting other areas.

The high number of fires occurring simultaneously in the span of just a few days in Oregon was fuelled by dry conditions, high temperatures and especially strong, swirling winds.

Ms Brown said on Thursday that more than 1,400 square miles have burned in Oregon over the past three days, nearly double the land that burns in a typical year in the state.

Oregon officials have not released an exact death count for the wildfires, but at least eight fatalities have been reported in the state.

A one-year-old boy was killed in wildfires in Washington.

A Northern California fire that tore through several hamlets in the foothills of the Sierra Nevada this week killed 10 people, making it the deadliest of the year.

Oregon’s congressional delegation announced on Friday that the White House has approved the state’s request for an emergency declaration that will help provide immediate assistance from the Federal Emergency Management Agency.