Tornadoes rip US country music capital, killing at least 22_1

Americans had as little as three minutes to brace for impact as an out-of-nowhere tornado barreled through Nashville and other parts of Tennessee, killing at least 22 people and injuring more than 150.

Buildings were flattened, cars were flipped upside down and some 50,000 houses and businesses had their electricity shut off when violent storms and several tornadoes struck at least four counties.

The death toll jumped to 22 after Governor William Lee declared a state of emergency, with many people still unaccounted for.

Despite the widespread destruction, polling sites at schools and elsewhere will be open for voting unless otherwise noted, officials said.

"We want people to exercise their rights and get out there and vote," governor Mr Lee said.

-with AAP