Washington  - 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."

The dam burst on Monday, releasing an estimated 5.4 million cubic metres of slurry into the Emory River and displacing 12 to 15 families, broadcast and newspaper reports said.

The 16-hectare pond was used by the Tennessee Valley Authority, one of the country's largest utilities, to store ash from the coal- burning Kingston Steam Plant in Harriman, Tennessee, about 80 kilometres west of Knoxville, Tennessee, a TVA spokesman told the Knoxville Journal.

The sludge was spread over at least 100 hectares, the newspaper reported. The Emory River feeds into the Clinch River, which empties into the Tennessee River.

While TVA officials said the spill had not affected drinking water, the Southern Alliance for Clean Energy (SACE) warned of undetermined consequences to water, land, residents and wildlife.

Stephen Smith, SACE's executive director, called for federal environmental officials and the TVA to encourage residents "to avoid bodily contact with the ash." Smith warned that the sludge contained lead and thallium. When the sludge dries, the toxins could become airborne, he warned in a statement on the group's website.

There are multiple pathways in which people can become potentially affected by these heavy metals, including bodily contact, drinking water, air pathways and aquatic wildlife and fish, and we feel that appropriate warnings should be expressed to ensure the safety of Tennessee residents," Smith said.

In an NBC news broadcast, one man stared out over the river of grey liquid sludge near his home, which until Monday looked out on a pristine body of flowing water.

"I've lost my past, my present and my future," he said.

The man said he had sent his family to Florida until the possible toxic content of the ash can be determined.

One video showed a large white frame house with a Christmas wreath outside the upper storey. The first floor was covered half-way in sludge.

Several people had to be rescued from collapsed homes.

Gilbert Francis Jr, a TVA spokesman, was quoted as saying by the New York Times that there was no evidence of toxic dangers.

"Most of that material is inert," said Francis said. "It does have some heavy metals within it, but it's not toxic or anything."

But environmentalists warned that as coal plants have reduced airborne toxic emissions, more such substances end up in the sludge byproducts, which are subjected to few regulations by state and federal authorities.

The New York Times said there were reports of large fish kills downstream from the spill. (dpa)