A wildland fire in north-central Oregon south of the Dalles has grown to an estimated 20,000 acres and is threatening dozens of homes and forcing hundreds to evacuate the area, officials said Thursday morning.

Strong winds from the northwest helped fuel the blaze, known as the South Valley fire, as it carved a path of destruction across wheat fields and stands of oak and ponderosa pine, the officials said during a press conference held in Dufur.

At one point Wednesday, as gusts reached 45 miles per hour, the fire managed to expand more than five miles in a matter of hours, the officials said.

Structures have been burned or badly damaged, though a precise figure is not known, said Stefan Myers with the Oregon State Fire Marshal's Office. Over 170 homes are threatened, Myers said at another press conference later Thursday, and over 500 people have been affected by evacuations.

The fire erupted Wednesday southwest of Dufur and was called in about 1:30 p.m. Officials believe the fire was likely human-caused, though the precise cause remains under investigation.

Governor Kate Brown Wednesday evening declared the blaze a conflagration, allowing the state fire marshal to order statewide firefighters and equipment to be used to assist in battling the fire.

Roughly 150 firefighters are currently battling the blaze, officials said. More than 100 local ranchers have also pitched in to help. Officials said the fire is 5 percent contained.

A section of U.S. 197 that was closed south of Dufur on Wednesday has since re-opened, according to TripCheck. The latest evacuation information can be found on the Wasco County Sheriff's Office's Facebook page.

A Red Cross shelter has been opened at The Dalles Middle School, said Christie Shaw, a spokeswoman with the Oregon Department of Forestry.

The South Valley fire is the third major wildfire in recent weeks to erupt in the area, which includes parts of Wasco and Sherman counties.

The 80,000-acre Substation fire ravaged fields, farms and families east of the Dalles last month and killed one rancher.

The Long Hollow fire near Dufur began last week and charred more than 30,000 acres, though it is now almost entirely contained.

-- Shane Dixon Kavanaugh

skavanaugh@oregonian.com

503-294-7632 || @shanedkavanaugh

Jim Ryan of The Oregonian/OregonLive staff contributed to this report