The National Weather Service (NWS) warned on Monday of a winter storm system carrying arctic air that would cause "record cold," with wind chill expected to hit as low as -60 degrees Fahrenheit (-51 degrees Celsius) in Montana and North Dakota.

"I've got more tough news. The death toll in Kentucky has now risen to 12," said Kentucky Governor Andy Beshear in a social media post on Monday, raising the toll from eight a day earlier.

West Virginia Governor Patrick Morrisey said on Monday his state had also seen at least one death from the weather.

"We have one confirmed fatality at this time," he told a press briefing, warning that further flooding was expected. "There are still several people who are missing."

In addition, one person died in the southern city of Atlanta, Georgia. The victim was killed when an "extremely large" tree fell on his house early Sunday, fire official Scott Powell told local media.

Most of the dead in Kentucky, Beshear said in an earlier news conference, drowned when trapped in their vehicles by fast-rising floodwaters. The victims included a mother and her child.

The governor urged people to stay off roads across the state, where local and federal authorities have declared a state of emergency.

Beshear said more than 1,000 people had been rescued by first responders within 24 hours.

In its Monday advisory, the NWS warned that the cold weather system would impact a vast area, sending temperatures tumbling in the central plains, the eastern seaboard and as far south as the Gulf coast.

"A bitter cold arctic airmass is expected to continue impacting the north-central US while also spreading further south and east over the next few days," the advisory said.

Power to thousands of homes had been restored by Monday, but more than 50,000 customers remained without electricity in the states of West Virginia, Pennsylvania and Maryland, according to monitoring website poweroutage.us.