Hurricane Dorian crept towards the southeast coast of the United States, weakening slightly but remaining a dangerous storm after leaving a trail of death and destruction in the Bahamas.

At least five deaths have been reported in the Bahamas from a storm which Prime Minister Hubert Minnis called a "historic tragedy" for the Atlantic archipelago.

The Miami-based National Hurricane Center (NHC) said Dorian, which has dumped as much as 76cm of rain on the Bahamas, had been downgraded from a Category 3 to a Category 2 storm on the five-level wind scale.

At 11.00am local time, it said Dorian was packing maximum sustained winds of 175km/ph, down from 185km/ph, the NHC said.

Dorian was located about 169km east of Fort Pierce, Florida, and moving in a northwesterly direction towards the Florida coast at a snail's pace of just over 3km/ph, it said.

The NHC said Grand Bahama Island was continuing to experience dangerous winds, life-threatening storm surge and extreme flooding from the heavy rain.

Eye of Hurricane Dorian captured by NASA astronaut Nick Hague on the ISS

The hazardous conditions would continue through much of today on Grand Bahama Island, which is the northernmost island in the archipelago and home to its largest city, Freeport, the NHC said.

It said Dorian was expected to pick up speed and grow in size during the day and turn towards the north tomorrow evening.

"The hurricane will then move dangerously close to the Florida east coast late today through Wednesday evening, very near the Georgia and South Carolina coasts Wednesday night and Thursday, and near or over the North Carolina coast late Thursday and Thursday night," the NHC said.

As Dorian ground to a standstill over the Bahamas, the tourism and aviation ministry announced the start of rescue operations "in parts where it is safe".

For many, the wait for help to arrive has been terrifying.

A text message seen by AFP from a woman who lives on Grand Bahama, said: "We are under water; we are up in the ceiling. Can someone please assist us or send some help. Please. Me and my six grandchildren and my son, we are in the ceiling."

Fear gripped residents of Freeport as winds tore off shutters and water began entering homes, another resident said by text from the Grand Bahama island's main city.

"People who thought they were safe are now calling for help. My best friend's husband is stuck in the roof of their house with seven feet (two meters) water below."

Initial Red Cross estimates were that 13,000 buildings may have been damaged or destroyed by Dorian in the Bahamas, officials in Geneva said.

Video posted on the website of the Tribune 242 newspaper showed water up to the roofs of wooden houses in what appeared to be a coastal town. Capsized boats floated in muddy brown water clogged with debris.

Florida has started to feel the effects of Dorian, with heavy rain and strong gusts of wind reported.

More than 9,500 people have taken cover in 121 shelters in Florida, according to the state's Division of Emergency Management.

Florida senator and former governor Rick Scott wrote on Twitter that "a slight wobble West" would bring the storm "on shore with devastating consequences."

"If you're in an evacuation zone, get out NOW. We can rebuild your home. We can't rebuild your life."

Orlando International Airport, one of the largest in the state, planned to cease commercial operations today because of the storm, it said in a statement.

Walt Disney World Resort in Orlando will also close.

South Carolina Governor Henry McMaster ordered mandatory evacuations for parts of eight coastal counties.

More than 830,000 people were under evacuation orders in Charleston and other coastal communities in the state, emergency management officials announced.

Georgia Governor Brian Kemp ordered evacuations in six coastal counties, including all of Savannah's 150,000 residents.

Virginia Governor Ralph Northam declared a state of emergency in his state, his office said, anticipating the southeast coast could be hit by the storm on Thursday.

Dorian was tied with Gilbert (1988), Wilma (2005) and the 1935 Labor Day hurricane for the second-strongest Atlantic hurricane on record, based on maximum sustained winds.

Allen in 1980 was the most powerful, with 306km/h winds, the NHC said.