A winter storm in the US that left millions without power in record-breaking cold weather has claimed more lives.

The death toll on Tuesday included three people found dead after a tornado hit a seaside town in North Carolina and four family members who perished in a Houston-area house fire while using a fireplace to stay warm.

The storm that overwhelmed power grids and immobilised the Southern Plains carried heavy snow and freezing rain into New England and the Deep South and left behind painfully low temperatures. Wind-chill warnings extended from Canada into Mexico.

At least 20 deaths were reported. Other causes included car crashes and carbon monoxide poisoning.

The weather also threatened to affect the nation’s COVID-19 vaccination effort. US President Joe Biden’s administration said delays in vaccine shipments and deliveries were likely.

The National Weather Service was “very surprised how rapidly this storm intensified … and at the time of night when most people are at home and in bed, it creates a very dangerous situation,” Emergency Services Director Ed Conrow said.

The worst US power outages were in Texas, affecting more than four million homes and businesses.

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In Texas, three young children and their grandmother died in the Houston-area fire, which likely began while they were using a fireplace to keep warm during a power outage, a fire official said.

Other Texas deaths included a woman and a girl who died from suspected carbon monoxide poisoning in Houston at a home without electricity from a car left running in an attached garage, and two men found along Houston-area roadways who likely died in sub-freezing temperatures, law enforcement officials said.

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In west Tennessee, a 10-year-old boy died after falling into an ice-covered pond on Sunday during a winter storm, fire officials said.

Several cities had record lows: In Minnesota, the Hibbing/Chisholm weather station registered minus 39 degrees. Sioux Falls, South Dakota, dropped to minus 26 degrees.

Air travel was also affected. At midday on Tuesday, more than 2700 US flights had been cancelled.

-AAP