The National Weather Service in Boulder issued a high-wind warning for much of northeastern Colorado, including Boulder County, where more than 6,000 acres had burned, The Denver Post said Friday.
Winds above 35 mph make unsafe flying conditions for pilots of tankers and helicopters fighting the Fourmile Canyon fire, officials said.
"We may not be able to use them," said Jim Thomas, the incident commander.
No one had been injured in the fire that started about 10 a.m. Monday when a vehicle collided with a propane tank in the canyon.
More than 700 firefighters from dozens of local, regional and national agencies have battled the fire, which has already cost more than $2.1 million to fight.
Firefighters got a bit of help Wednesday when .08 inches of rain fell, but its benefit was quickly offset when the wind picked up, the report said.
The fire is being attacked around the clock with the help of specially trained night firefighters.
The fire was about 30 percent contained Thursday, Thomas said.
Residents of about 800 homes over 8,000 acres were evacuated, the newspaper said.