A desperate search for survivors is under way in parts of six US states devastated by powerful tornadoes that have left at least 83 people dead.

Dozens more people are missing and entire towns were destroyed by about 30 tornadoes on Friday.

President Biden has declared a disaster in Kentucky, the worst-affected state.

More than 70 people have died in the state, including dozens in a candle factory, and the death toll is expected to rise above 100.

Local congressman James Comer, working with rescuers in the ruined town of Mayfield, said the tornado there was the widest ever seen.

"It's the most devastating storm damage I've seen in my entire life. We've had tornadoes that have been the same length as this tornado but we've never had one with the width of this tornado," he said.