Residents across the United States have begun cleaning up the aftermath of Hurricane Helene. (AP PHOTO)

Authorities across a wide swath of the southeastern United States are facing the daunting task of cleaning up from Hurricane Helene, one of the most powerful to hit the country, as the death toll continues to rise.

At least 64 deaths were reported by Saturday evening, and officials feared still more bodies would be discovered across several states.

Downgraded late on Friday to a post-tropical cyclone, the remnants of Helene continued to produce heavy rains across several states, sparking massive flooding that threatened to cause dam failures that could inundate entire towns.

“The devastation we’re witnessing in Hurricane Helene’s wake has been overwhelming,” US President Joe Biden said on Saturday.

“Jill and I continue to pray for all of those who have lost loved ones and for everyone impacted by this storm.”

“Our focus is on life-saving and life-sustaining response efforts in the southeast,” he said.

“I continue to be briefed by my team, which is co-ordinating recovery efforts with state and local officials.”

In Florida’s Pinellas County near Tampa, Sheriff Bob Gualtieri said he had never seen destruction like what was wrought by Helene.

“I would just describe it, having spent the last few hours out there, as a war zone,” he told a press conference.

At least 3.25 million people woke up early on Saturday without power across five states, with authorities warning it could be several days before services were fully restored.

The worst outages were in South Carolina with more than one million homes and businesses without power, and Georgia with 777,000 without power.

As of 11am the remnants of the storm was about 235km southwest of Louisville, Kentucky.

Some of the worst rains hit western North Carolina, which received almost 76cm of precipitation on Mount Mitchell in Yancey County, the NWS’s Weather Prediction Center reported.

Atlanta was hit with 33cm of rain, and in South Georgia farmers were assessing the damages to the state’s $US1 billion ($A1.4 billion) cotton crop now in its harvest season.

Before moving north through Georgia and into Tennessee and the Carolinas, Helene hit Florida’s Big Bend region as a powerful category four hurricane on Thursday night, packing 225km/h winds.

It left behind a chaotic landscape of overturned boats in harbours, felled trees, submerged cars and flooded streets.

Police and firefighters carried out thousands of water rescues throughout the affected states on Friday.

More than 50 people were rescued from the roof of a hospital in Unicoi County, Tennessee, about 200km northeast of Knoxville, state officials said, after flood waters swamped the rural community.

Rising waters from the Nolichucky River prevented ambulances and emergency vehicles from rescuing patients and others there, the Unicoi County Emergency Management Agency said on social media.

Emergency crews in boats and helicopters were used to conduct rescues.

Elsewhere in Tennessee, Rob Mathis, the mayor of Cocke County, ordered the evacuation of downtown Newport because of a potential failure at the nearby Walters dam.

The National Weather Service issued flash flood warnings overnight for a swath of eastern Tennessee covering 100,000 residents, warning them to seek higher ground.

The Nolichucky Dam in Tennessee’s Greene County was on the brink of failure early on Saturday, officials reported, adding that a breach could occur at any time.