Witnesses said the huge twister destroyed several dozen trailer homes and houses when it touched down near Benton, a town of 2,000 about 10 miles north of Shreveport in northwestern Louisiana at 4:30pm (11.30pm BST).
"This is a terrible, terrible disaster," the local sheriff, Larry Deen, said. "I've lived here all my life and there's never been anything like this."
Chuck Mazziotti, director of the Office of Emergency Preparedness for Bossier Parish, said six deaths were confirmed at the hardest-hit trailer park and officials were trying to confirm reports of three more deaths near Cypress Lake.
"There may well be more." he said.
"The half-mile-square mobile home park was just destroyed, and until we get someone who worked there, we don't know how many trailers there were or the number of people in them.
"We don't even know how many people we're looking for. We've got mobile homes piled on top of each other two and three deep." (Reuters)