Fire investigators say a 20-acre Butte wildfire in the Mat-Su Valley was sparked by a single campfire that was neither permitted nor constructed properly.

Norm McDonald, a fire management officer with the Alaska Division of Forestry's Mat-Su Valley office, says the blaze began with an unpermitted fire which didn't have a required barrier, such as a ring of rocks, to separate it from nearby elements.

The Butte Fire Department and Division of Forestry were using all-terrain vehicles, a helicopter and hot shot crews Wednesday to fight a fire that started around 10:30 p.m. that evening and had since burned an area near Maud and Plumley Roads, McDonald said.

The influx of effort and an initial attack by fire helicopters was able to prevent new growth in the blaze, apparently confining it to 21 acres separated by about two miles from nearby homes. Another crew was expected to arrive from McGrath Thursday afternoon.

With thick grayish-white smoke billowing over the Knik River Valley and the fire pushing uncomfortably close to residential areas, firefighters had hoped to extinguish the blaze Thursday.

The National Weather Service has issued burn warnings due to the recent dry spell across Southcentral Alaska and much of the rest of the state.

Woodland firefighter Renette Saba says that while the fire didn't initially threaten homes, the possibility remains on crews' minds.

"No structures were immediately threatened but it did have potential (due to) the black spruce component," Saba said. "The fuel type is incredibly volatile, especially with the high winds we had yesterday morning."

McDonald says the fire is 75 percent contained as of Thursday evening, with crews working through the night in a bid to fully contain it by Friday morning.

Channel 2's Dan Carpenter, Amberia Hill and Chris Klint contributed information to this story.