A furious storm system that kicked up tornadoes, flash floods and hail as big as grapefruits has claimed at least 45 lives across the southern United States.

Emergency crews spent the weekend searching for victims in hard-hit swaths of North Carolina, where 62 tornadoes were reported from the worst spring storm in two decades to hit the state. Authorities warned the death toll was likely to rise further Sunday as searchers probed shattered homes and businesses.

The storms claimed its first lives Thursday night in Oklahoma, then roared through Arkansas, Mississippi, Alabama and Georgia. They hit North Carolina and neighbouring Virginia on Saturday before the sprawling, potent storm bands moved eastward over the Atlantic.

In North Carolina, Gov. Beverly Perdue declared a state of emergency and said the 62 tornadoes reported were the most deadly since March 1984, when a storm system spawned 22 twisters in the Carolinas that killed 57 people and injured hundreds.

"Our thoughts and prayers are with everybody in North Carolina who has been through this horrible day," Perdue said.

Authorities in North Carolina said they would provide more details of the death toll later Sunday after checking on the reports of fatalities in at least four counties and in the capital city, Raleigh. Search and rescue teams operated through the night, Perdue said, with damage assessments starting in earnest Sunday after dawn.

Police and rescue crews began conducting house-to-house searches later Saturday at a mobile home park in north Raleigh, where the storm snapped some trees in half, ripped others out of the ground and tossed some trailers from one side of a street to the other.

In Virginia, disaster officials said one apparent tornado ripped more than 19 kilometres through Gloucester County, along the Chesapeake Bay, uprooting trees and pounding homes to rubble while claiming three lives. One person was killed in flash flooding elsewhere in the state, and another was missing.