Emergency workers struggled to reach victims as search and rescue operations continued into Wednesday and the scope of the damage and humanitarian crisis unfolded.

"We are in the midst of one of the greatest national crises in our country's history," Bahamas Prime Minister Hubert Minnis told a news conference. "No effort or resources will be held back."

News media reported early on Wednesday that some storm victims remained stuck on rooftops, waiting for rescue. The official death count of seven is expected rise in the coming days.

"We can expect more deaths to be recorded. This is just preliminary information," Minnis told a news conference.

"Marsh Harbor has suffered, I would estimate, in excess of 60 per cent damage to their homes," Minnis said, referring to the port on Great Abaco.

"The Mud, as we know, has been completely destroyed or decimated," he said referring to a shantytown known as the Mud and the Peas.

Aerial video of the Bahamas' Great Abaco Island revealed kilometres of flooded neighbourhoods, pulverised buildings, upturned boats and shipping containers scattered like toys. Many buildings had walls or roofs partly ripped off.

"Victims are being loaded on flatbed trucks across Abaco," said one Twitter poster with the handle @mvp242, describing a rain-blurred photograph of limp bodies strewn across a truck bed. Other Twitter messages said whole communities were swept away.

Dorian's winds had diminished to a Category 2 storm on the five-step Saffir-Simpson intensity scale, the hurricane grew in size and picked up speed.

Forecasters said it would come dangerously close to Florida's east coast on Wednesday, where more than a million people have been ordered evacuated.

Dorian packed sustained winds of 165km/h and was moving northwest as it churned about 170km east of Daytona Beach, Florida, the National Hurricane Center said in a 3am advisory.

Hurricane-force winds had expanded to 96 km from the storm's core. "Dorian is expected to remain a powerful hurricane during the next couple of days," the NHC said.

Forecasters said that tornadoes were possible overnight on the Florida coast and that risk would later shift to Georgia and South Carolina.

With telephones down on Abaco and Grand Bahama islands, residents posted lists of missing loved ones across social media.

The exact toll in the Bahamas will not be clear until the storm passes and rescue crews can get to devastated areas, said Theo Neilly, the Bahamian consul general in Washington.

"We expect it to be very devastating and the damage to be extreme," Neilly said.

As many as 13,000 homes in the Bahamas may have been destroyed or severely damaged, the International Federation of Red Cross and Red Crescent Societies said, in the strongest storm ever to hit the Bahamas.

Dorian, which killed one person in Puerto Rico before striking the Bahamas on Sunday, is tied for the second-strongest Atlantic storm to make landfall with Gilbert (1988), Wilma (2005) and the 1935 Labor Day hurricane.

It was expected to hit Florida overnight, before bringing its powerful winds and dangerous surf to the coasts of Georgia, South Carolina and North Carolina by late on Thursday. More than one million people were ordered to evacuate coastal counties in those states.

Australian Associated Press