The swirling Tennessee coal plant sludge is flooding homes, forcing evacuation of the residents of Kingston.

Five days after an earthen dam in Tennessee burst and spilled millions of cubic metres of coal ash slurry down a river valley, families were still evacuating and environmentalists Friday called it an "historical energy disaster.

Estimates for the amount of the thick sludge have tripled to more than a billion gallons, as cleanup crews are fighting hard to halt its oozing into an adjacent river.

A spokesman for Tennessee Valley Authority, a federal corporation and the US's largest public power company, said the agency has never experienced a spill of this magnitude.

TVA's initial estimate for the spill was 1.8 million cubic yards or more than 360 million gallons of sludge. By Friday, the estimate reached 5.4 million cubic yards or more than 1 billion gallons, enough to fill 1,660 Olympic-size swimming pools.

The plant sits on a tributary of the Tennessee River called the Clinch River. At least 300 acres of land has been coated by the sludge, a bigger area than the Exxon Valdez oil spill.

Environmental advocates say the ash contains concentrated levels of mercury and arsenic.

The sludge, a byproduct of the ash from coal combustion, was contained at a retention site at the Tennessee Valley Authority's power plant in Kingston, about 40 miles east of Knoxville. The retention wall breached early Monday, sending the sludge downhill and damaging 15 homes.