"The bottom line is that there is increasing confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity," Blake said.

By Monday morning (local time), Florence had top sustained winds of 185 km/h. It was centred about 2,000 kilometres east-southeast of Cape Fear, North Carolina, and moving west at 20 km/h. Its centre will move between Bermuda and the Bahamas on Tuesday and Wednesday, and approach the coast of South Carolina or North Carolina on Thursday, the National Hurricane Centre said.

Hurricanes Isaac, which could hit Caribbean islands, and Helene, much farther out to sea, lined up behind Florence as the 2018 Atlantic season reached its peak.

Preparations intensified up and down the densely populated coast.

On North Carolina's Outer Banks, Dawn Farrow Taylor, 50, was gathering photos and important documents and filling prescriptions on Monday before heading inland. She grew up on the shore, and says this will be only the second time she's evacuated.

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Since reliable record-keeping began more than 150 years ago, North Carolina has only been hit by one Category 4 hurricane: Hazel, with 210 km/h winds in 1954.

"I don't think many of us have ever been through a Category 4. And out here we're so fragile. We're just a strip of land - we're a barrier island... Already we're getting some overwash, the ocean is coming over 12," she said, referring to the islands' main road.

The governors of North and South Carolina and Virginia declared states of emergency far ahead of the approaching storm. South Carolina Governor Henry McMaster also suspended his campaign and asked President Donald Trump for a federal emergency declaration.

Navy ships off Virginia's coast were preparing to sail out of the path, a North Carolina university has already cancelled classes and people have begun stocking up on plywood, bottled water and other supplies.

Red flags have already been flying on beaches, warning swimmers to stay out of the water as seas began kicking up. People rushed to get emergency kits ready, map out escape routes, fill sandbags and secure their homes.

"Pretend, assume, presume that a major hurricane is going to hit right smack dab in the middle of South Carolina and is going to go way inshore," South Carolina Governor Henry McMaster said. The state's emergency management agency said it is "preparing for the possibility of a large-scale disaster".

In coastal Charleston, South Carolina, city officials offered sandbags to residents. Myrtle Beach Mayor Brenda Bethune urged people to secure their homes but said it was too early to know if evacuations will be ordered.

Myrtle Beach hardware stores and supermarkets were busy ringing up sales of bottled water, plywood and generators.

"Literally, they are filling buggies full of water, shopping carts full of water," Ryan Deeck, grocery department manager at a Walmart, told The Sun News. "They're coming in and buying water and plates, and that's about all they're buying."

North Carolina crews were getting bulldozers and chain saws ready.

Florida-based Carnival Cruise Line was re-routing its cruise ships, but there were other hurricanes to contend with.

Lining up behind Florence, Isaac was about 1,855 kilometres east of the Windward Islands with top winds of 120 km/r early on Monday, moving west at 22 km/h. Isaac's forward movement was accelerating on a path to cross into the lower Caribbean on Thursday.

Helene, meanwhile, was still in the Atlantic's spawning ground for hurricanes off the coast of Africa, swirling with 165 km/h winds and forecast to become a major hurricane, about 600 kilometres west of the Cape Verde islands.