By RUSS BYNUM, Associated Press

SAVANNAH, Ga. (AP) — A wildfire sparked by lightning spread rapidly Monday inside the Okefenokee National Wildlife Refuge, where officials estimated roughly 6 square miles had burned near the Georgia-Florida state line.

Low humidity allowed the blaze to jump from 940 acres Sunday to about 3,800 acres Monday, said Sarah Wyatt, a refuge ranger and spokeswoman. She said fire crews were using bulldozers to plow fire breaks in an effort to contain the blaze within the refuge, which covers nearly 407,000 acres of swampy terrain in southeast Georgia.

"The conditions are just extremely dry," Wyatt said. "When everything is dried out like that, it's going to catch on fire."

The fire posed no immediate threat to people, homes or other structures, Wyatt said, adding all trails and camp sites inside the refuge remain open to visitors. The blaze was burning in the southern portion of the refuge near the Georgia-Florida state line.

Thick smoke in the area caused authorities Monday to close a stretch of Florida state Highway 2 because of poor visibility, said Lt. Aaron Stephens of the Florida Highway Patrol.

Wyatt said swamps need to burn periodically so they won't become overgrown and turn into uplands. As long as people and property aren't in danger, she said, it's standard practice to let the fire burn inside the refuge.

The refuge, which is headquartered in Folkston about 120 miles southwest of Savannah, is managed by the U.S. Fish and Wildlife Service. Large fires in the swamp can burn for months before they run out of fuel.

In April 2011, lightning was blamed for a fire in the refuge that burned and smoldered for nearly a full year. The blaze charred a total of 309,200 acres, or about 483 square miles, before authorities declared it was extinguished. Only 18,206 acres burned outside the Okefenokee refuge. No homes were lost and no serious injuries were reported.

Georgia officials are bracing for a busy wildfire season this year as unseasonably dry weather covers most of the state. Much of central and southern Georgia, including the Okefenokee Swamp, is considered abnormally dry, according to the U.S. Drought Monitor. And a large portion of northern Georgia is suffering from moderate to extreme drought.