Seven people were killed in Tennessee and four in northern Mississippi by storms that brought heavy flooding and tornadoes to the region over the weekend.
More storms were looming as emergency officials in Tennessee asked for help from the state's Army National Guard and urged people to stay off flooded roads.
At a news conference, Tennessee Governor Phil Bredesen said it will be days before the floods recede enough to assess the damage to roads and bridges.
Tennessee officials have confirmed seven deaths. At least three people are missing after getting swept away by flood waters, and one of them is presumed dead by the Tennessee Emergency Management Agency, which would bring the death toll to eight.
Meanwhile, National Guard and Tennessee Highway Patrol helicopters with hoisting capabilities were on standby to rescue people from the flooding that left cars stranded on interstate highways.
In northern Mississippi, officials said a man died after his car was submerged under flood waters while two people died in a mobile home.
County Emergency Management coordinator David Shaw said one person was killed in the town of Abbeville, where 15 or 20 houses were damaged by strong winds.
A line of strong thunderstorms dumped at least 10 inches (25 centimetres) of rain on Memphis and produced tornadoes and hail along the Mississippi River Valley in Mississippi, Arkansas, Missouri and Kentucky.
Forecasters predicted more rain which could halt rescuers trying to reach all of the far-flung areas that have been affected.
Some areas were hit by 13 inches (33 centimetres) of flash flooding on Saturday, and the same on Sunday.