Hundreds of firefighters, aided by helicopters dropping fire retardant and water, battled two large wildfires that threatened to merge near the most populated part of the US state of Oregon, including the suburbs of Portland.

The number of people ordered to evacuate statewide because of fires rose to an estimated 500,000, more than 10% of the state’s 4.2 million people, the Oregon Office of Emergency Management reported late on Thursday.

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

A change in the weather, with winds dropping and shifting direction and humidity rising, greatly helped firefighters struggling to prevent two fires, one burning southeast of Portland and the other east of Salem, the state capital, 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, 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 said.

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

Governor Kate Brown said 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 and an area greater than the size of Rhode Island.

Oregon officials haven’t released an exact death count for the wildfires, but at least four fatalities have been reported in the state.

One person was killed in wildfires in Washington.

Hundreds of thousands of people were ordered to flee encroaching flames.

The estimate of evacuees was calculated by determining how many people live in mandatory evacuation zones, Office of Emergency Management spokeswoman Bobbi Doan said.

One fire approached Molalla, triggering a mandatory evacuation order for the community of about 9,000 located 30 miles south of Portland.

A police car rolled through the streets with a loudspeaker blaring “evacuate now.”

With the two large fires, called the Beachie Fire and the Riverside Fire, threatening to merge, some firefighters in Clackamas County, which encompasses Molalla, were told to disengage temporarily on Thursday because of the danger.

Officials tried to reassure residents who abandoned their homes and law enforcement officials said police patrols would be stepped up to prevent looting.

The local fire department said on Twitter: “To be clear, your firefighters are still working hard on the wildfires in Clackamas County.

“They are taking a ‘tactical pause’ to allow firefighters to reposition, get accountability & evaluate extreme fire conditions.”

“We haven’t abandoned you,” the fire officials said.

The change in weather also aided efforts to contain a fire near Lincoln City, on the Oregon Coast.

“Thank God, we got a wind shift. The wind started coming from the west, pushing the fire back towards the east, and that’s what kept it within its footprint and kept it from growing,” fire spokesperson Ashley Lertora said.