Since it began with a lightning strike in Virginia's Tidewater area Aug. 4, the fire has burned more than 6,300 acres and is the largest ever at the refuge, The Washington Post reported.
The blaze is on pace to become the longest-burning fire since the swamp, located in southeastern Virginia and northeastern North Carolina, was designated as a refuge in 1974, the newspaper said.
Swamp fires rarely endanger people and property but federal officials say their thick smoke can be a health concern.
In the early days of the Virginia fire, officials received numerous phone calls from worried residents, refuge manager Chris Lowie said.
"Citizens of Hampton Roads were ... saying, 'When is the fire going to be out?' 'I'm tired of the smoke.' 'I have asthma.' 'I can't go outside.' Things like that," he said.
Swamp fires burn in peat, dead leaves and vegetation formed over thousands of years, Lowie said.
Unlike wildfires in grasslands or forests, swamp fires in peat burn four to six feet underground, making it hard for firefighters to reach, he said.