The United Nations said 70,000 people in the Bahamas needed immediate humanitarian relief after the most damaging storm ever to hit the island nation.

Aerial video of the worst-hit Abaco Islands in the northern Bahamas showed widespread devastation, with the harbour, shops, workplaces, a hospital and airport landing strips damaged or decimated, frustrating rescue efforts.

One of the most powerful Caribbean storms on record, Dorian was rated a Category 5 hurricane when it killed at least 20 people in the Bahamas.

Authorities expect that number to rise, Prime Minister Hubert Minnis said at a news briefing, as retreating floodwaters revealed the scope of destruction.

"My island of Abaco, everything is gone. No banks, no stores, no nothing," said Marsh Harbour resident, Ramond A. King as he surveyed the wreckage of his home, which had its roof ripped off and debris scattered throughout. "Everything is gone, just bodies."

With telephones down in many areas, residents posted lists of missing loved ones on social media. One Facebook post by media outlet Our News Bahamas had 2500 comments, mainly listing lost family members.

Dorian killed one person in Puerto Rico before hovering over the Bahamas for two days with torrential rains and fierce winds that whipped up 3.7 to 5.5-metre storm surges.

South Carolina was preparing for a record storm surge, potentially reaching a height of two metres at the popular vacation destination of Myrtle Beach, the US National Hurricane Center said in an advisory.

The storm was located about 115 km south-southeast of Charleston, South Carolina, at 8am EST on Thursday and was headed north-northeast at about 13km/h, the NHC said. It was packing 185 km/h winds.

An international relief effort was underway for the Bahamas, with a British Royal Navy vessel providing assistance and Jamaica sending a 150-member military contingent to help secure Abaco and Grand Bahama, officials said.

Volunteers also ferried supplies to the islands in a flotilla of small boats.

"Let us give of our best in this moment of historic tragedy," Minnis said.

He also encouraged international tourists to visit the Bahamas, which relies heavily on its hospitality industry.

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.

Norwegian energy company Equinor said it had discovered a spill on the ground outside tanks at its damaged storage terminal in the Bahamas, but was unclear on volumes and had not seen any oil at sea.

The State Department said it did not believe any US citizens who were in the Bahamas during the storm were killed.

President Donald Trump said the United States was sending supplies, including materials originally intended for any Dorian victims in Florida.

Australian Associated Press