A fire burning out of control for more than a day in Emigration Canyon was rated Saturday as the highest priority fire in the Mountain West, the U.S. Forest Service said.

Firefighters began trickling in Saturday afternoon from around the nation to fight the blaze that has charred more than 3,000 acres, threatened about 50 homes and filled the Salt Lake Valley with smoke.Meanwhile, firefighters also battled three other blazes in Utah.

Ed Storey, a Forest Service spokesman, said the Emigration Canyon fire has become the most important among wildfires burning in Utah and parts of Idaho, Wyoming and Nevada.

The fire even outranks blazes in Yellowstone National Park, Kathy Jo Pollock, Interagency Fire Center spokeswoman, said.

"At the moment this is a higher priority fire because structures are threatened," Pollock said.

Saturday morning the fire was fueled by high southwest winds. Shifting winds later Saturday made it difficult to keep flames from reaching 45 to 50 homes in the Killyon Canyon and Pinecrest areas.

"The fire just goes all kinds of different ways, so it is hard to keep a finger on it," Pollock said.

The winds blew a cloud of smoke and ash into Salt Lake City, reducing visibility at the Salt Lake International Airport from the usual 40 to 50 miles to only 10 to 12 miles, the National Weather Service said.

David Carpenter, lead forecaster, said visibility was only five miles at the Provo airport and even less in other parts of the valley.

Late Friday and early Saturday the Killyon Canyon fire jumped across a road and a creek and then moved northwest up Burr Fork toward Pinecrest. Flames had reached within 1,000 yards of 10 homes in the Killyon Canyon area. The fire had tripled in size overnight, officials said.

Some 400 firefighters were expected to arrive in the state by Monday from lower-priority fire lines in Northeastern and Southern states, Storey said. About 80 had arrived by Saturday night, bringing the total number of firefighters to 200.

The majority of the firefighters were working near the Pinecrest and Killyon Canyon homes, Pollock said.

Storey said the crews would help supplement air drops of chemical retardant that continued Saturday from four tankers and one helicopter.

"That retardant by itself is not effective. We have to have hand crews on the ground to put lines around those retardant drops," Storey said.

About 60 engines - ranging from fire trucks from around Salt Lake Valley to 250-gallon, all-terrain pumpers - were being used to battle the fire. Caterpillars that arrived at the fire early Saturday were used to help build firebreaks around the threatened homes. Steep terrain kept the tractors from some areas.

The fire spread from a fire line on the east side of the Burr Fork road across Killyon Canyon into the Little Mountain area. Flames were kept from traveling down Red Butte Canyon, but smoke from the fire shrouded the canyon area above the University of Utah.

Pollock said the fire moved to within two miles of I-80 at Parley's Summit Saturday afternoon. A Utah High Patrol dispatcher said that so many people slowed or stopped to look at the fire, traffic almost came to a standstill. UHP troopers were sent to keep people from parking or slowing.

Officials were still investigating the cause of the fire, but said an unattended campfire was among possibilities.

Meanwhile, Uinta National Forest officials are hoping to have a 350-acre fire near Alpine, Utah County, contained by Monday and controlled by Tuesday, "if the wind cooperates," forest spokeswoman Loyal Clark said.

Winds caused three new fingers of the fire to creep up the north ridge in Fort Canyon just above Alpine and crews were concerned that the entire ridge could be consumed if predicted 45 mph winds blow through the canyon.

"Right now we have people posted at the base camp, and all they are doing is looking to see when the fire flares up on the ridge so that we can send the resting crews in to fight," Clark said.

Helicopters dumped water on the new flare-ups throughout Saturday afternoon until nightfall.

The Federal Aviation Administration imposed an air closure on aircraft over the fire area to discourage curious spectators from the air.

Clark said million-dollar homes in the canyon were probably out of danger because the fire changed direction early Saturday afternoon.

Forest officials believe the blaze outside Alpine was sparked Wednesday afternoon by an abandoned campfire above Sliding Rock in Fort Canyon. Forest Service officials reminded Labor Day campers that the forest is closed to fires and heavy fines will be levied on anyone caught starting an illegal fire.

A fire near Powder Mountain ski resort in Weber Canyon continued to thwart firefighters. Projections that the fire would be contained by Sunday were dropped after the fire burned 3,000 more acres, Pollock said.

About 90 firefighters battled the Powder Mountain fire, about 26 miles east of Ogden. Pollock said fire-fighters were uncertain they would be able to meet their Monday containment goal.

"We're not sure that's going to be done because the wind is playing tricks there, too," she said.

The blaze was started more than a week ago by a spark from a bulldozer's exhaust. At one point, it threatened the small Utah town of Eden, but Pollock said the wind shifted and drove the fire away from the town.

"That fire is no longer a threat to the town of Eden," she said. "The wind today turned that fire to the east and to the south and there are no structures up in there."

A third fire burned near Walls-burg, near Deer Creek Reservoir, and had blackened more than 160 acres by Saturday afternoon.