GROVELAND, Calif. >> With winds gusting to 50 mph on Sierra mountain ridges and flames jumping from treetop to treetop, hundreds of firefighters have been deployed to protect mountain communities in the path of a fire raging north of Yosemite National Park.

Overnight the fire grew 7 square miles as firefighters gained little ground in slowing the now 207-square-mile blaze, said Daniel Berlant of the California Department of Forestry and Fire Protection.

“Today, unfortunately, we are expecting strong winds out of the south,” he said. “It’s going to allow the fire to advance to the northeast.”

Fire officials are using bulldozers to clear contingency lines on the Rim Fire’s north side to protect the towns of Tuolumne City, Ponderosa Hills and Twain Hart. The lines are being cut a mile ahead of the fire in locations where fire officials hope they will help protect the communities should the fire jump containment lines.

The high winds and movement of the fire from bone-dry brush on the ground to 100-foot oak and pine treetops have created dire conditions.

“A crown fire is much more difficult to fight,” Berlant told The Associated Press today. “Our firefighters are on the ground having to spray up.”

Officials estimate containment at just 7 percent.

The blaze sweeping across steep, rugged river canyons quickly has become one of the biggest in California history, thanks in part to extremely dry conditions caused by a lack of snow and rainfall this year. Investigators are trying to determine how it started Aug. 17, days before lightning storms swept through the region and sparked other, smaller blazes.

The fire is the most critical of a dozen burning across California, officials say. More than 12 helicopters and a half-dozen fixed wing tankers are dropping water and retardant from the air and 2,800 firefighters are on the ground.

“This fire has continued to pose every challenge that there can be on a fire: inaccessible terrain, strong winds, dry conditions. It’s a very difficult firefight,” Berlant said.

Statewide more than 8,300 firefighters are battling nearly 400 square miles of fires. Many air districts have issued health advisories as smoke settles over Northern California. On Saturday organizers cancelled the 24th annual Lake in the Sky Air Show at Lake Tahoe because of poor visibility.

The Rim Fire has threatened two groves of giant sequoias that are unique the region, prompting park employees to clear brush and setting sprinklers.

The towering trees, which grow only on the western slopes of the Sierra Nevada and are among the largest and oldest living things on earth, can resist fire. However, dry conditions and heavy brush are forcing park officials to take extra precautions in the Tuolumne and Merced groves.

The U.S. Forest Service says about 4,500 structures are threatened by the Rim Fire. Berlant said 23 structures were destroyed, though officials have not determined whether they were homes or rural outbuildings.

The tourist mecca of Yosemite Valley, the part of the park known around the world for such sights as the Half Dome and El Capitan rock formations and waterfalls, remained open, clear of smoke and free from other signs of the fire that remained about 20 miles away.