Hurricane Dorian, the second strongest Atlantic storm on record, has pounded the Bahamian islands of Great Abaco and Grand Bahama and is forecast to move dangerously close to Florida in coming days, US forecasters say.

Eyewitness News in the Bahamas reported late on Sunday (local time) that an eight-year-old boy had been killed in the Abaco Islands.

Ingird McIntosh told the news website her daughter had told her own two children were missing. The boy’s body was later found; it is thought he drowned.

There have been power outages across the islands as they have weathered Dorian, and communication lines are cut in and out.

Grand Bahama Island had earlier been warned to expect “catastrophic” storm surges up to seven metres above normal tide levels, with higher destructive waves, the Miami-based US National Hurricane Centre said.

“This is a life-threatening situation. Residents on Grand Bahama Island should not leave their shelter when the eye passes over as winds will rapidly increase on the other side of the eye,” the centre said in an advisory note.

“Residents in the Abacos should … stay in their shelter until conditions subside later today.”

Forecasters said Dorian might spare the US coast a direct hit, but would track up Florida’s east coast from Monday and affect the South Carolina coast on Wednesday or Thursday. More than 20 million people are likely to feel the storm’s effects within a few days.

Even a glancing blow from one of the strongest storms ever to menace Florida could bring torrential rains and damaging winds, the NHC said.

“On this track, the core of extremely dangerous Hurricane Dorian will continue to pound Great Abaco and Grand Bahama islands tonight and Monday,” the NHC said.

Looking down through the eye has produced amazing images of the structure of the storm. You can also see the outline of islands in the Bahamas as #Dorian crawls west. This is the Grand Bahama island, and it is taking a huge hit from the eyewall now. pic.twitter.com/ohGxzHJq99

— Daniel J. Tomaso (@DopplerDan) September 2, 2019

Dorian’s wind speeds had slowed slightly from 295km/h per hour to 290km/h but it remained a category five hurricane.

“Slow weakening is forecast, but fluctuations in intensity could occur couple of days. Regardless, Dorian is expected to remain a powerful hurricane during the next few days,” a late-night advisory from the NHC read.

Late on Sunday, South Carolina Governor Henry McMaster ordered mandatory evacuations for parts of eight coastal counties. In Georgia, Governor Brian Kemp made the same order for six coastal counties.

William Snyder, the sheriff of Florida’s Martin County, urged residents to heed warnings from authorities.

He urged anyone in a mobile home, manufactured home or areas prone to flooding to relocate.

“We are within 20 miles [32 kilometres] of an apocalyptic hurricane coming ashore on the Treasure Coast,” he said.

“If the storm wobbles 20 miles or 30 miles off its current path, and that’s not a lot, we will have landfall.”

enquanto a mídia noticia que o furacão Dorian esta para chegar na Flórida nessa segunda feira, ele já devastou completamente as Bahamas 🥺💔 pic.twitter.com/kUBfHIzN8E

— dom que xote das necas de pitibiribas (@i0gedes) September 2, 2019

Dorian is forecast to remain a hurricane for the next five days, the NHC said. It made landfall on Elbow Cay in the Abaco Islands with maximum sustained winds of 295km/h and gusts of more than 354km/h.

It was the strongest hurricane on record to hit the north-western Bahamas as a life-threatening category five storm on the five-step Saffir-Simpson Wind Scale.

US President Donald Trump also urged Americans to get out of the way of the storm.

“We don’t even know what’s coming at us. All we know is it’s possibly the biggest. I have – I’m not sure that I’ve ever even heard of a category five. I knew it existed. And I’ve seen some category fours – you don’t even see them that much,” Mr Trump said at a briefing with officials in Washington, DC.

“But a category five … That’s the ultimate, and that’s what we have unfortunately.”

In fact, Dorian is one of four category five hurricanes to hit parts of the US since Mr Trump became president.

Plywood covers the windows of an art gallery in Titusville, Florida. Photo: AAP

On Sunday, petrol stations around Titusville, on Florida’s east coast, were closed. Many grocery stores were open but boarded up. Inside, shelves emptied out fast.

Like many officials in Florida, Governor Ron DeSantis warned residents against becoming complacent after forecasts began saying on Saturday that the state might escape a direct hit.

“This storm at this magnitude could really cause massive destruction. Do not put your life in jeopardy by staying behind when you have a chance to get out,” Mr DeSantis said, urging people to heed evacuation orders from county officials.

Dozens of Floridians who live in boats in marinas in Brevard County were rushing to secure vessels, strapping them to docks and removing canvas coverings from decks. However, some boat residents said they wanted to ride out Dorian.

Residents on Abaco posted video on social media showing floodwaters halfway up the sides of single-family homes with parts of the roofs torn off. Car alarms blared across the island, which was littered with twisted metal and splintered wood.

Forecasters predicted up to 76 centimetres of rain. The storm was moving at just 7km/h on Sunday night, prolonging a pummelling expected to last for hours, the NHC said.

Bahamian Prime Minister Hubert Minnis said in a nationally televised news conference that a “deadly storm and a monster storm” was battering the islands.

Homes there are built to withstand winds of at least 240km/h, but the expected storm surge is higher than the average roof.

“This will put us to a test that we’ve never confronted before,” Mr Minnis said. “This is probably the most sad and worst day of my life to address the Bahamian people.”

Also on Sunday, a new tropical storm formed south-west of Mexico. It was expected to become a hurricane on Monday, the NHC said.

Tropical Storm Juliette was 735 kilometres from Manzanillo, Mexico, with maximum sustained winds of 95km/h, the NHC said.

-with AAP