Devastating hurricane yet to hit US coast

Weather warnings for Florida have been extended further north as Hurricane Dorian stays almost stationary off the coast of The Bahamas, bring damaging winds and massive flooding.

Forecasters expect Dorian to stay off shore, but meteorologist Daniel Brown cautioned that "only a small deviation" could draw the storm's dangerous core toward land.

By 10 pm EDT Monday (12pm Tuesday AEST), the storm's top sustained winds had fallen to 220 km/h, still within Category 4 range.

It was still virtually stationary, centred just 40 kilometres northeast of Freeport in The Bahamas - about the same distance from the city it had been 10 hours earlier.

Hurricane force winds extended outward as far as 75 kilometres from the centre

Dorian is likely to begin pulling away from the Bahamas early Tuesday and curving to the northeast parallel to the southeastern coast of the US.

The system is expected to come within 64 to 80 kilometres off the Florida coast, with hurricane-force wind speeds extending about 56 kilometres to the west.

An advisory from the hurricane centre warned that Florida's east-central coast could see a brief tornado sometime Monday afternoon or evening.

A mandatory evacuation of entire South Carolina coast took effect Monday covering about 830,000 people.

Transportation officials reversed all lanes of Interstate 26 from Charleston to head inland earlier than planned after noticing traffic jams from evacuees and vacationers heading home on Labor Day, Governor Henry McMaster said.

"We can't make everybody happy, but we believe we can keep everyone alive," the governor said.

A few hours later, Georgia's governor, Brian Kemp, ordered mandatory evacuations for that state's Atlantic coast, also starting at midday Monday.

Authorities in Florida ordered mandatory evacuations in some vulnerable coastal areas. North Carolina Governor Roy Cooper warned his state that it could see heavy rain, winds and floods later in the week.

A hurricane watch was in effect for Florida's East Coast from Deerfield Beach north to South Santee River in South Carolina. A storm surge watch was extended northward to South Santee River in South Carolina. Lake Okeechobee was under a tropical storm watch.

A National Guard official, John Anderson, said many people were complying with the evacuation orders.

"We have not seen much resistance at all," he said in a phone call with reporters. People do understand that Dorian is nothing to mess around with."

President Donald Trump approved emergency declarations for the states of Georgia and South Carolina because of Dorian.

The declarations authorise the Federal Emergency Management Agency to co-ordinate disaster relief efforts.

The president urged everyone in Dorian's path to obey warnings and evacuation orders issued by local authorities, saying public safety "must always come first".

Dorian battered the Bahamas on Monday with life-threatening winds and storm surge even as it weakened very slightly to a Category 4 storm.

The Bahamas Press reported on Twitter that a boy had drowned in the northern Bahamas, the first recorded fatality of Dorian, and four others are dead.

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.