States of emergency have been declared in North Carolina, South Carolina and Virginia as residents were told to "pretend, assume, presume" Hurricane Florence will make it to land from the Carribbean.
Experts have forecast the upgraded tropical storm will gather force - going from a category 1 hurricane to a category 3 on Monday morning.
The US National Hurricane Center has warned that winds could reach 130 mph, turning Florence into a fearsome category 4 storm.
It is too early to know the exact path the storm will take but it could hit land on Thursday, the centre added.
Navy ships off Virginia's coast are getting ready to sail out of the path of the powerful hurricane and one North Carolina university has cancelled classes.
People are already filling stockpiling plywood, bottled water and other supplies.
"Literally, they are filling buggies full of water, shopping carts full of water," a Walmart manager told local media in Myrtle Beach, South Carolina.
Hurricane Florence is around 500m north of the Leeward Islands, which includes four Commonwealth nations - Saint Kitts, Nevis, Barbuda and Antigua.
Henry McMaster, governor of South Carolina, warned the US should be braced for the hurricane to hit.
"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," he said.
Even if a direct blow is avoided, if Hurricane Florence slows or stalls near the shore it could cause devastating inland flooding, by unloading a foot or two of rain.
Forecasters have warned of a life-threatening storm surge, along with the damage of a hurricane's high winds.
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