Red flags flying on beaches warned swimmers to stay out of waters already roiled by the distant storm, and cruise ships and Navy vessels were set to be steered out of harm's way. People rushed to buy bottled water, plywood and other supplies.

Florence crossed the 74 mph threshold from tropical storm to hurricane Sunday morning, and by evening its winds were up to 85 mph (140 kph) as the National Hurricane Center warned it was expected to become an extremely dangerous major hurricane by Monday and remain that way for days.

As of 5 p.m. EDT, Florence was centered about 720 miles (1,160 kilometers) southeast of Bermuda, moving west at 7 mph (11 kmh). Drawing energy from the warm water, it could be a fearsome Category 4 with winds of 130 mph (209 kph) or more by Tuesday, the Miami-based center said.

Forecasters said it is too early to know the exact path the storm will take but warned that it could roll ashore in the Carolinas by Thursday.

Forecasters urged residents from South Carolina to the mid-Atlantic to get ready — and not just for a possible direct blow against the coast. They warned that Florence could slow or stall after coming ashore, with some forecasting models showing it could unload a foot or two of rain in places, causing devastating inland flooding. Forecasters also warned that the threat of a life-threatening storm surge was rising.

"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 Gov. Henry McMaster said. The state's emergency management agency said it is "preparing for the possibility of a large-scale disaster."

In Charleston, South Carolina, along the coast, city officials offered sandbags to residents. Myrtle Beach Mayor Brenda Bethune urged people to secure their homes but said it's 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 officials started getting bulldozers and chain saws ready.

Across the Southeast, people were urged to put together emergency supply kits, prepare their homes and research evacuation routes. The governors of North and South Carolina and Virginia declared states of emergency far ahead of the storm to get ready.

Florence's effects were already being felt along the coast, with dangerous swells and rip currents in some spots. On North Carolina's Outer Banks, the town of Nags Head posted no-swimming flags on beaches.

In Jacksonville, North Carolina, about 20 miles (32 kilometers) from the coast, some residents picked up hurricane supplies during their normal weekend shopping, The Daily News reported. Ilija Cesal told the newspaper he wouldn't worry about buying extra water or other supplies for a few more days.

"I'll see by Wednesday how that goes — we got over 48 hours before that happens," Cesal said.

In southeast Virginia, Naval Station Norfolk told its employees they should not leave their vehicles parked at the sprawling base in coming days because of the flood threat. The station, the largest naval complex in the world, said in a Facebook post that much of the base is prone to heavy flooding, especially the parking lots adjacent to the waterfront.

The Navy planned to send ships from the Hampton Roads area of Virginia out to sea. Florida-based Carnival Cruise Line re-routed its cruise ships.

As Hurricane Florence approaches the East Coast, the University of North Carolina at Wilmington encouraged its students to leave campus for a safer location. The university said Sunday that it has issued a voluntary evacuation for students starting at midday Monday, noting classes would be canceled.

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Bynum reported from Savannah, Georgia. AP writers Sarah Rankin in Richmond, Virginia, and Gary Fineout in Tallahassee, Florida, contributed to this story.