A wildfire in northern Idaho has destroyed at least six homes and forced about 200 residents in an upscale lakeside community to evacuate as it ballooned to more than 3 square miles Monday.

The blaze started Sunday afternoon and burned through a mix of year-round residences and seasonal homes near the southern shore of Lake Pend Oreille, the state's largest lake and a recreation and fishing destination. The flames have destroyed other buildings such as boat houses and pump houses in the Bayview area that includes expensive homes, especially next to the water.

About 100 firefighters on the ground put in a fire line between the fire and homes, but another 200 residents have been told to be ready to evacuate if needed, fire spokesman Jim Lyon said.

"We're dealing with some extremely steep terrain," he said.

The fight included four aircraft and a U.S. Navy boat that was dousing homes, but crews faced temperatures expected around 90 degrees Monday, with winds picking up in the afternoon. Those are the same conditions as on Sunday when the fire grew, Lyon said.

The blaze started near the lake shore and burned upslope through timber, Lyon said. He said a team of investigators planned to examine where the fire started to try to determine a cause.

"We have a good idea of where it started," he said. "There were people out on a boat that saw it. It started down low at lake level, or near the edge of the lake, and traveled up and southwesterly."

No injuries have been reported from the fire burning through a mix of private, state and federal land.