Some 750 firefighters, five airplanes and eight helicopters are helping to fight the blaze, which has spread across nearly 80,000 hectares.

Threat to Hanford

On Wednesday, the fire briefly threatened a Hanford building holding radioactive waste. The flames then leapt over the Yakima River to burn homes near Benton City.

We're all still a little leery here

John Bickford, Hanford Information Centre

"The fire is considered 40% contained on the Hanford site," he said.

However, firefighters were concerned that winds forecasted to blow at up to 50 km/h (30 mph) could push the flames to the dry brush closer to the nuclear areas.

Firefighters have taken to the air to protect the Hanford complex

"That's why we're all still a little leery here."

The reservation was set up during the 1940s to process plutonium for the Manhattan Project, which built the first atomic bombs used on Japan in World War II.

A massive radioactive waste clean-up project was recently launched.

Heart of America Northwest, a watchdog group following the Hanford clean-up project, said it was concerned the fire could sweep dangerous levels of plutonium into the air in certain areas.

"If the fire reaches contaminated radioactive soil, it could create serious radiation health hazards," the group said.

Radiological crews monitoring the area had as yet found no unusually high levels of radiation.

The incident comes shortly after another wildfire, which threatened the most important US nuclear facility at Los Alamos in May.