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Jackson Hole News & Guide, Bradly J. Boner) MANDATORY CREDIT; MAGS OUT; NO SALES, Associated Press
A DC-10 air tanker drops its load of fire retardant as the the Horsethief Canyon Fire continues to burn near Jackson, Wyo., on Wednesday, Sept. 12, 2012.

CHEYENNE, Wyo. — More favorable weather conditions allowed firefighters to be more aggressive Wednesday in battling wildfires threatening Casper and Jackson.

The Sheep Herder Hill Fire has burned about 25 square miles near Casper in central Wyoming, and the Horsethief Canyon Fire has burned about 2,700 acres near Jackson in the northwest part of the state.

Thanks to cool temperatures and light winds, firefighters were able to intentionally set fire to about 100 acres of unburned areas within the Horsethief Canyon Fire perimeter Wednesday afternoon, denying the fire fuels if weather conditions turn for the worse, fire spokeswoman Jenny Anzelmo-Sarles said.

"They're doing that in areas where there would be a greater chance for the fire to spread from and cross the fire line," Anzelmo-Sarles said. "So areas where it could jump and then run up a slope toward Snow King Mountain."

Fire managers were throwing 22 aircraft and nearly 300 firefighters at the Casper Mountain Fire, fire spokeswoman Laura McConnell said.

"They do have a couple of areas of concern, but they said overall it's looking good," she said.

The fire was about 10 percent contained Wednesday.

Firefighters planned to check the burned area for additional structures that may have been damaged or destroyed, McConnell said.

The fire has destroyed at least seven homes since it started Sunday on Casper Mountain, which overlooks the city of Casper. Another 750 residences were threatened.

About 400 residents have been forced to leave their homes. Some stayed overnight at an American Red Cross shelter set up in Casper.

In northwest Wyoming, firefighters were trying to burn vegetation to deny the Horsethief Canyon Fire new fuels.

The fire is burning south of the resort town next to Snow King Mountain, a ski hill that borders the town. It was also about 10 percent contained Wednesday.

About 1,000 residents were on alert to be prepared to leave in case the fire gets too close although officials reduced the size of the alerted area Wednesday. No evacuation orders have been issued and no structures have been lost. Tourist activities in Jackson have been unaffected.

The fire has been ruled human-caused but remains under investigation.

Anzelmo-Sarles said residents of the area have overwhelmed firefighters with food donations.

"Firefighters have been offered enough cookies that the cookies could put out the fire," she said. "It's highly appreciated, but it's a little overwhelming right now."

A better alternative would be to donate money to a local charitable organization, she said.