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.