At least 30 people across six states have been killed by a vicious storm system that unleashed dozens of tornadoes and was threatening to cause more damage in heavily populated regions of the U.S. South on Tuesday.

The hardest hit states have been Arkansas and Mississippi where 23 people have been killed and more than 200 injured by tornadoes over the past three days that have flattened neighborhoods, reduced homes to splinters and snapped trees like twigs.

Deaths were also reported in Oklahoma and Iowa on Sunday, and Alabama and Tennessee on Monday.

A large cluster of thunderstorms pelted Alabama, Georgia and the Florida Panhandle on Tuesday, with tornadoes likely touching down along the Alabama-Georgia state line, forecasters said.

"The main severe threat today is down in the central and eastern Gulf Coast," National Weather Service meteorologist Brynn Kerr said, adding there was potential for severe conditions in Tennessee and the Ohio Valley later on Tuesday.

Tens of thousands of customers along the path of the storm were without power on Tuesday morning, with the worst outages in parts of Alabama and Georgia, utility companies reported.

In western North Carolina, fire department personnel used boats to rescue people from homes and vehicles deluged overnight by flash floods.

In Arkansas, residents of central Faulkner County, where most of the damage occurred, sorted through the rubble as they tried to piece their lives back together.

"There is joy because you find something that's not broken and then you find something that's shattered that meant a lot," said Terry Lee, whose home was damaged by a tornado.

Georgia Governor Nathan Deal, who declared a state of emergency late on Monday in preparation for the looming storms, said in a statement, "We're prepared now, and we'll be ready for recovery should we, God forbid, experience tornado damage or flooding."

In Tupelo, Mississippi, which was hit by a tornado on Monday, police were going house to house searching for victims and trying to stem any gas leaks that could fuel fires.

Officials were also picking through the rubble in Lincoln County, Tennessee, near the Alabama state line late Monday evening, where a tornado touched down, killing two people.

One of the victims in Alabama was John Servati, 21, a member of the University of Alabama's swim team who died in a friend's basement when a retaining wall collapsed, a university spokeswoman said.