SONOMA, Calif. — The death toll rose to 21 Wednesday and more than 3,500 homes and businesses have been destroyed across Northern California's wine country as almost two dozen wildfires continued their assault on the region for a fourth day Wednesday.

The California Department of Forestry and Fire Protection said fire activity increased significantly overnight, destroying more buildings and forcing more mandatory evacuations.

"Yesterday was a very aggressive day for fire expansion," Cal Fire's deputy commander Barry Biermann said. "We had a lot of wind."

Air rescue teams have plucked more than 50 people from roofs and mountaintops. Chris Childs, with the California Highway Patrol, said pets have been included in the rescues. He also cautioned that all evacuees must remain patient before returning to their homes.

"I know it's a tough message to be told you cannot go back to your homes, but I ask for your patience as you deal with officers at those road closures," Childs said.

More than 250 square miles have burned since the fires began igniting Sunday. Authorities have yet to determine the cause of the fires, but they say the weather has complicated efforts to fight them.

The toll has been brutal: At least 21 dead, thousands of homes, businesses and other buildings destroyed. Sonoma and Napa counties have taken the brunt of the damage.

Sonoma County Sheriff Rob Giordano said 11 deaths have been confirmed in his county alone.

"When we start doing searches, I would expect that number to go up," Giordano said."The devastation is enormous."

Giordano said more than 600 missing persons reports have been filed. He said the vast majority were due to cellphone issues, including spotty service and drained phone batteries as residents packed and fled in a hurry.

He urged people in evacuation zones to leave sooner, not later.

“People underestimate how powerful this can be,” Giordana said.

Dean Knight fled his home in the mountains of Glen Ellen for a familiar shelter at Sonoma Valley High School. Knight taught chemistry at the school for 46 years. Since Sunday he has been passing out water and facemasks and otherwise helping out in any way he can.

He also can lend an empathetic ear to some of his 250 fellow evacuees. Some talk of losing everything. All he knows about his own home is that neighbors said it was still standing Monday.

"Whatever happens, happens," he said. "The main thing is people got out. ... We're keeping our fingers crossed."

Fog and relatively cool temperatures descended on the hard-hit area Wednesday, but the National Weather Service had little good news for the 4,000-plus firefighters battling the blazes for a fourth day.

"No rainfall is forecast for ongoing fires in California," the weather service said. "Strong winds behind the front will bring elevated-to-critical fire weather threats to active fires across northern California today into Thursday."

Contributing: Elizabeth Weise; The Associated Press