Residents in the southern United States are bracing for more severe thunderstorms following tornadoes on Tuesday that killed dozens of people in Tennessee.

A slow-moving system is likely to bring more heavy rain along with the threat of major storms to a region that was plagued by heavy rain and flooding last month.

The peak of the latest rain event is expected late on Wednesday into Thursday and locals are on alert for dangerous flash floods.

The region from Texas to South Carolina is expected to see the worst of the rain, with anywhere from 50mm to 150mm accumulating in the coming days.

The National Weather Service (NWS) has issued flash flood watches from eastern Louisiana into central and southern Mississippi, much of Alabama, Georgia and the western Florida panhandle.

As well as the threat of tornadoes, the storms are forecast to bring damaging winds and large hail.

More than 80 river gauges across the south are still reporting levels above flood stage, with this additional heavy rain threatening to prolong existing river flooding and trigger new dangerously high levels elsewhere.

Meanwhile, on Tuesday, the NWS-Nashville began surveying the damage from tornadoes that hit the city of Nashville and surrounding areas in Tennessee, killing at least 25 people.

The actual number of tornadoes remains unknown, but the NWS in Nashville confirmed that at least one EF3 tornado, with winds of 220 kilometres (137 miles) per hour to 265km/h (165m/h), hitting the Nashville metro area.

"Looks like it's quite possible we had one or maybe two, long-track tornadoes across Davidson, Wilson and Smith counties," NWS-Nashville said on Twitter on Tuesday evening.

"In the coming days, we will determine if these tornado paths are actually one or two tornadoes."

With a further survey of Putnam County still to take place, the actual number of tornadoes might not be confirmed until the end of the week.