Motorists in the US have streamed inland from the east coast on highways that turned into one-way routes as more than one million people in three states were ordered to get out of the way of Hurricane Florence.

The hair-raising storm is taking aim at North and South Carolina with 210 km/h winds and potentially devastating rains.

Florence was expected to blow ashore late on Thursday or early on Friday, then slow down and wring itself out for days.

It’s tipped to unload 30-60 cm of rain that could cause flooding well inland and wreak environmental havoc by washing over industrial waste sites and pig farms.

Forecasters and politicians pleaded with the public on Tuesday to take the warnings seriously and minced no words in describing the threat.

“This storm is a monster. It’s big and it’s vicious. It is an extremely dangerous, life-threatening, historic hurricane,” North Carolina Governor Roy Cooper said.

North Carolina residents prepare for the arrival of Hurricane Florence. Photo: Getty

“The waves and the wind this storm may bring is nothing like you’ve ever seen,” Mr Cooper said.

“Even if you’ve ridden out storms before, this one is different. Don’t bet your life on riding out a monster.”

North and South Carolina and Virginia ordered mass evacuations along the coast. But getting out of harm’s way could prove difficult.

Florence is so wide that a life-threatening storm surge was being pushed 480 km ahead of its eye, and a swath of states from South Carolina to Ohio and Pennsylvania could get deluged.

People across the region rushed to buy bottled water and other supplies, board up their homes or just get out of town.

A line of heavy traffic moved away from the coast on the main thoroughfare between the port city of Wilmington and inland Raleigh, becoming gridlocked in places.

Only a trickle of vehicles was going in the opposite direction, including pickup trucks stocked with plywood and other building materials.

Florence has turned into a category 4 hurricane as it approaches the US coast. Photo: NASA

Service stations started running out of petrol as far west as Raleigh, with bright yellow bags, signs or rags placed over the pumps to show they were out of order.

At 2pm local time the storm was centred 1360 km southeast of Cape Fear, North Carolina, moving at 28 km/h.

It was a potentially catastrophic category 4 storm but was expected to keep drawing energy from the warm water and intensify to near category 5, which means winds of 253 km/h.

“This one really scares me,” National Hurricane Centre Director Ken Graham warned.

Forecasters said parts of North Carolina could get 50 cm of rain, if not more, with as much as 25 cm predicted for elsewhere in the state and in Virginia, parts of Maryland and Washington DC.

Florence could slam the Carolinas harder than any hurricane since Hazel, which hit in 1954 with 209 km/h winds.

The category 4 storm destroyed 15,000 buildings and killed 19 people in North Carolina.

-AAP