Thick smoke rose out of the smoldering floor of the Great Dismal Swamp yesterday and drifted across the dirt road where firefighters were trying to stop the blaze.

Wayne Johnson, an information officer for the U.S. Forest Service, warned that the woods were full of snags -- burned-out trees that fell on the unwary long after the fire had passed. As he spoke, a big tree hidden in the smoke hit the earth with a crash.

Johnson is one of more than 360 firefighters trying to hem in the South One fire, a 2,700-acre blaze in the remote swamp between Lake Drummond and the North Carolina line. Over the past two weeks, shifting winds have sent the fire's smoke for hundreds of miles in every direction.

Along the smoky fire lines yesterday, authorities agreed the swamp fire could burn for weeks, possibly longer, as the flames creep through the rich, mulchlike soil that covers everything.

With heavy equipment, they leveled foliage and widened firebreaks they hope eventually will contain the fire within a 5,600-acre rectangle. They sprayed water onto steaming hot spots. They hoped for heavy rain and against heavy winds, which they said could abruptly turn a stubborn fire nasty.

"Things are going pretty well, but there's a lot of fuel around, and there's still a lot of potential in this fire," said Johnson, watching flames flicker inside a blackened tree stump.

Joe Smith, a Forest Service supervisor, said the fire creeps through the soil and then abruptly grows when it finds prime fuel, such as fallen trees. A few minutes later, a brush pile erupted briefly into 12-foot flames.

Refuge officials said the fire started June 9 when a logging contractor's equipment caught fire in an area littered with trees that fell to Hurricane Isabel's winds in 2003. The fire fed on the fallen trees and the tinder-dry duff and marsh grass, and spread quickly through uninhabited woods and marsh.

Rather than send firefighters on the attack in the dense, flammable swamp, their bosses sketched a 5,600-acre border around the fire, along existing roads and ditches where firefighters had access to vehicles and water. The plan is to allow the fire to expand to that border, and stop it there, Smith said.

The southern fire line, for example, is Corapeake Ditch Road, a dirt road with a ditch of water running alongside. Firefighters used heavy equipment and backfires to clear foliage from both sides of the road. They hope the resulting gap is wide enough that the fire will not be able to jump it and spread south.

Two nights ago, a flaming ember from a backfire floated across the road and started a spot fire that took all the next day to contain.

Near the fire's northern border, firefighters are widening the fire line while helicopters drop buckets of water on the advancing flames to give the firefighters more time.

Along the eastern edge of the fire, where winds have spread it the fastest in recent days, firefighters are preparing a fire line along a ditch, but they have set up two alternate lines farther back in case the fire crosses the first.

The swamp fire was roughly 25 percent contained yesterday. How long it will take to extinguish is unclear, particularly with no significant rain in the forecast, Johnson said.

Thunderstorms dropped as much as a half-inch of rain on some parts of the fire, he said, but in the thick woods, "the rain hardly reached the ground. We need 3 or 4 inches of rain."