Powerful Hurricane Earl bore down on the east coast of the US on Thursday on a path toward North Carolina's barrier islands, which is expected to lash with dangerous winds and pounding waves.

The US National Hurricane Centre said the Category 4 storm has strengthened with top winds reaching 145mph.

Earl's centre, located about 355 miles south of Cape Hatteras on Thursday morning, is expected to pass near North Carolina's Outer Banks by the evening before gradually tracking up the east coast on Friday.

While a direct hit is not forecast, Earl is on track to deliver a sideswipe to the North Carolina coast.

"Folks should not focus on the exact track because this is a very large hurricane," said Hugh Cobb, a meteorologist with the Miami-based hurricane centre.

"Hurricane force winds extend out 90 miles from the centre," Cobb said. "So even if it doesn't make either a direct landfall, or even come within 60 miles of the Outer Banks, they're still going to get hurricane force winds just because of the sheer size of the storm."

Large swells are buffeting the coastline as experts warn Earl will bring dangerously high seas.

North Carolina officials responded by expanding a mandatory evacuation order across new areas of the state's low-lying barrier islands.