Gusting overnight winds at a federal laboratory whipped what started as a small southeastern Idaho desert fire Tuesday into a blaze that by Wednesday afternoon covered 170 square mile and had become the biggest wildfire burning in the nation.

About 300 firefighters from the Idaho National Laboratory and the Bureau of Land Management are on scene with 28 fire engines, a helicopter, nine bulldozers, five air tankers and an air attack plane.

"It's a wind-driven range fire and they can gobble up a lot of territory in a very short time," said Don Smurthwaite, a Bureau of Land Management spokesman at the National Interagency Fire Center. "That's what's happening."

The fire has charred power lines, causing a loss of electricity to the laboratory's Materials & Fuels Complex, where research and development of nuclear fuels takes place, officials said.

The complex, about 28 miles west of Idaho Falls, is being powered by diesel generators.

Officials said no "critical infrastructure" had burned.

John Epperson, a laboratory spokesman in Idaho Falls, said winds had died Wednesday. Crews had contained about 40 percent of the fire on the laboratory site by late Wednesday, but were still monitoring hotspots. It's still unclear just how the wildfire started, he said, adding it appears to have ignited Tuesday near a road that's closed to the public, but accessible to employees.

"Conditions were clear, with high winds," Epperson said. "There wasn't any obvious sign of lightning."

The fire, about seven miles northeast of the 25-person hamlet of Atomic City, has now burned an area equal to about a fifth of the 890-square-mile site that has served as a federal U.S. Department of Energy nuclear research facility since 1949.

The 700 employees who work at the Materials & Fuels Complex have been told to work from another location, or if that's not possible, to stay home.

Any structures that have burned are likely outbuildings or sheds, Epperson said, but smoke is obscuring visibility and fire crews are concentrating on fighting the fire, not assessing damage. INL areas with hazardous waste have been protected by fire lines, noncombustible containment materials and fire suppression systems.

Officials said firefighters with bulldozers are focusing primarily on the north front between laboratory site and State Highway 33 and on the west front.

The National Interagency Fire Center, which coordinates the national wildfire response from its headquarters in Boise, said the fire is the largest of nine active wildfires burning in seven states, which include Alaska, Arizona, California, Idaho, Montana, New Mexico and Washington.