A car drives through a flooded street in Nassau, Bahamas. (Reuters)

Hurricane Dorian smashed parts of the tourism-dependent Bahamas to ruins, leaving relief officials scrambling on Tuesday to battle an unfolding humanitarian crisis as the scale of the catastrophe begins to emerge.

Aerial video of the Bahamas' Great Abaco Island revealed mile upon mile of flooded neighborhoods, pulverized buildings, upturned boats and shipping containers scattered like Lego toys.

Many buildings had walls or roofs partly ripped off.

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

Forecasters said it would come dangerously close in the next 36 hours to Florida's east coast, where more than a million people have been ordered evacuated.

Bahamas Prime Minister Hubert Minnis put the death toll at seven.

"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 percent 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 in reference to a shantytown known as the Mud and the Peas.

"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.

Minnis said he saw people waving for help in a community near Coopers Town on Great Abaco, after it was cut off by floods.

"There were around 30 people trapped and waving yellow flags, sheets and shirts to bring our attention to their survival."

A video posted on Twitter showed a storm surge rising up inside a two-story home, the sofa and other furniture floating toward the second floor. Another showed residents trying to swim from one home to another through the surge.

In another, a woman repeatedly says, "Please pray for us," after the storm ripped the roof off her apartment building, exposing her, and other residents, to the elements as she struggled to shelter her 4-month-old baby.

"Some people the water just sucked them," she said. "Some people didn't make it."

Reports from Grand Bahama island, and its main town of Freeport, have been more sketchy. The weather has made it impossible for relief agencies to get through, Minnis said.

The Miami-based US National Hurricane Center said storm surges in Grand Bahama were 12 to 18 feet (3.7 to 5.5 m) above normal tide levels.

LONG LISTS OF MISSING

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

A single Facebook post by media outlet Our News Bahamas seeking the names of missing people had 1,600 comments listing lost family members since it went live on Tuesday morning.

Dorian packed sustained winds of 110 miles per hour (175 kph) and was moving northwest at 6 mph (9 kph), as it churned about 95 miles (155 km) east of Cape Canaveral, Florida, the NHC said in an 11 p.m. EDT (0300 GMT) advisory.

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

Hurricane-force winds had expanded to 60 miles (96 km) from the storm's core. "Dorian is expected to remain a powerful hurricane during the next couple of days," the NHC said.

The exact toll in the Bahamas will not be clear until the storm completely 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. Dorian has battered the Bahamas for the past three days.

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.

Food may be required for 14,500 people in the northern Bahamas' Abaco Islands and for 45,700 people in Grand Bahama, the U.N. World Food Programme said in a statement. The preliminary estimates were based on an assessment by representatives of Caribbean nations, the WFP and other groups.

The US Agency for International Development said on Twitter it was airlifting critical relief items, such as plastic sheeting, hygiene kits, and water containers, from Miami to the Bahamas. The US Coast Guard said four of its helicopters were assisting in humanitarian efforts.

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 with hurricane conditions overnight, before bringing its powerful winds and dangerous surf along the coasts of Georgia, South Carolina and North Carolina by late on Thursday.

The governors of those states have ordered evacuations of coastal counties.

President Donald Trump declared a state of emergency for South Carolina on Tuesday, freeing funds, other federal resources and manpower to assist during the storm and aftermath recovery.

Emergencies have already been declared in Florida and Georgia.