Authorities say mud is the latest challenge facing firefighters battling a blaze in the Great Dismal Swamp National Wildlife Refuge.

Annaleasa Winter is a spokeswoman for the multi-departmental team of more than 200 firefighters who are fighting the fire, which has burned 4,664 acres since it started June 9.

Winter says five days of rain have cooled the swamp's surface but muddy conditions have kept some vehicles from accessing the fire site.

The fire remains 90 percent contained with virtually no visible flames, but layers of peat continue to smoke and smolder. Firefighters are focusing on keeping the area drenched.

The refuge covers 110,000 acres in Virginia and North Carolina.