(CNN) A northern California wildfire continued to rage Friday, threatening homes and torching 6,900 acres in Napa and Sonoma Counties, authorities said.

The two-day-old fire roared so fiercely that satellite imagery captured its fiery footprint near Lake Berryessa, the National Weather Service said.

Napa and Sonoma Counties are part of California's wine country, but one Napa operator of wine tours said the fire posed no threats to vineyards on their tour.

"Our thoughts are with the families impacted by the Wragg Fire near Lake Berryessa, which is an hour's drive from our station," the Napa Valley Wine Train said on its Facebook page. The business offers a three-hour tour aboard an antique train through famous vineyards.

"Wine Train operations will continue as scheduled, unaffected by the fire," the firm said.

Our thoughts are with the families impacted by the Wragg Fire near Lake Berryessa, which is an hour's drive from our... Posted by The Napa Valley Wine Train on Thursday, July 23, 2015

More than 1,500 fire personnel gained a little more control of the fire Thursday night, increasing containment to 20% as of Friday morning, said the California Department of Forestry and Fire Protection, or Cal Fire.

Evacuations remained in place for at least two communities, Quail Ridge and the Canyon Creek Area, but earlier-evacuated residents were allowed to return to two other areas, in Golden Bear Estates and along County Road 87, said Cal Fire.

Take a look at this satellite image from last night. #WraggFire showing up brightly on this map. #cawx pic.twitter.com/jSJewWaHiY — NWS Sacramento (@NWSSacramento) July 23, 2015

A large center was opened in Winters for the evacuation of animals in the rural area, authorities said.

A former horse boarding facility owner opened his doors to any evacuated horses: http://t.co/07R2QbNDVE #WraggFire pic.twitter.com/tyv38szbeS — ABC7 News (@abc7newsBayArea) July 24, 2015

Only one outbuilding and a tent structure have been destroyed since the fire began Wednesday, but 150 structures remained threatened Friday, authorities said.

An initial report claimed that a vehicle accident sparked the fire, but the cause remained under investigation Friday, Cal Fire said.

"Firefighting personnel continue to aggressively work in treacherous terrain, providing structure defense and improving containment lines. All road closures will remain closed. The terrain is brush covered, rugged and steep, with difficult access," Cal Fire said in a statement.