Forecasts call for between 25 and 38 centimetres of rain in the hardest-hit areas, possibly more if the storm stalls over land, as expected, Graham said.

Virginia's emergency operations chief, Jeffrey Stern, told reporters that residents should brace for "something that no one in Virginia has experienced in their lifetimes".

North Carolina Governor Roy Cooper told a news conference his state was in "the bull's eye".

Cooper and his counterparts in South Carolina, Virginia and Maryland all declared states of emergency.

Jittery residents in the Carolinas have begun the rituals of disaster preparation - boarding up windows and stocking up on groceries, water and petrol.

South Carolina Governor Henry McMaster has ordered an estimated one million coastal residents in his state to move inland.

Mandatory evacuation orders also were issued for more than 300,000 people in North Carolina and Virginia.

US President Donald Trump has cancelled a political rally planned for Friday in Jackson, Mississippi, over safety concerns related to Florence, his campaign said.

Trump said in a Twitter message he had spoken to the governors of North and South Carolina and Virginia, adding: "Federal government stands by, ready to assist."

In Holden Beach, North Carolina, in the storm's path, long-time residents were busy preparing.

A local service station ran out of petrol for about two hours on Monday as motorists rushed to fill up their tanks.

"People are getting frantic," Assistant Manager Nadine Schrock said, adding her shop also sold out of cases of bottled water.

The US military said it was sending an advance team to Raleigh, North Carolina, to coordinate with state and local authorities.

It said about 750 military personnel would be designated to provide support.

The US Navy said it was sending nearly 30 ships from coastal Virginia out of port to ride out the storm.

Reuters