An "extremely dangerous" super typhoon predicted to be the one of the strongest systems on record was last night tearing towards Hong Kong and the Philippines with up to 43 million people in the firing line.

At the same time, authorities in the United States warned that while Hurricane Florence had been downgraded to a Category 1 storm, it was still likely to trigger life-threatening floods. Florence had last night already inundated coastal areas of North and South Carolina with ocean water and left tens of thousands without power. Forecasters said conditions would only worsen as the hulking storm moved inland.

States of emergency have already been declared in the Carolinas, Georgia, Virginia, Maryland and the District of Columbia.

Bureau of Meteorology Australia tropical climatologist Greg Browning said that Typhoon Mangkhut was equivalent to a Category 5 severe tropical cyclone and had maximum sustained winds of 205km/h and gusts of up to 285km/h. It was "significantly stronger" than Hurricane Florence, he said, which began flooding the Carolinas yesterday as the massive, slow-moving storm crept toward the coastline, threatening millions of people in its path with record rainfall and punishing surf.

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Browning said Mangkhut was "relatively rare at the top of the severe scale". "It's extremely dangerous as it's a very large system with very strong winds and a potential storm surge over a large distance.

"There will be very heavy rainfall associated with it which has potential to cause widespread damage."

More than 4 million people are at risk from the storm and Philippine authorities were yesterday evacuating thousands of people from its path, closing schools, readying bulldozers for landslides and placing rescuers and troops on full alert in the country's north.

Mangkhut was initially expected to hit the northern tip of Cagayan province tomorrow, but it's now likely to make landfall further south and closer to Isabela province. Philippine state forecaster Chris Perez said it was likely to then cut across the northern breadbasket. He said the change wouldn't make much difference because of the typhoon's massive size.

Hurricane Florence whipped up large waves and caused flooding in coastal areas in North Carolina ahead of making landfall. Photo / AP
Hurricane Florence whipped up large waves and caused flooding in coastal areas in North Carolina ahead of making landfall. Photo / AP

A huge raincloud band 900km wide, combined with seasonal monsoon rains, means the typhoon will bring heavy to intense rains that could set off landslides and flash floods. Warnings have been raised in 25 provinces across the main northern island of Luzon, restricting sea and air travel.

Browning said Mangkhut was the most powerful storm system to have developed on Earth this year but that it wasn't the strongest since records began in 1946, as has been reported. Typhoon Haiyan - which killed more than 6000 people when it lashed the Philippines with maximum sustained winds of 230km/h and gusts of 325km/h in 2013 - holds that record.

After the Philippines, the Hong Kong Observatory predicts Mangkhut will plough into the Chinese mainland on Monday south of Hong Kong and north of the island province of Hainan. Though it will weaken from a super typhoon to a severe typhoon, it will still be packing sustained winds of 175km/h.

On the Chinese mainland, the three southern provinces of Guangdong, Guangxi and Hainan are co-ordinating preparations, including suspending transport and moving people to shelter inland, the national meteorological agency reported.

Philippine President Rodrigo Duterte asked Cabinet officials from the north to help oversee disaster-response work if needed, and told reporters it was too early to consider seeking foreign aid. "It would depend on the severity of the crisis," Duterte said. "If it flattens everything, maybe we need to have some help."

Florence, meanwhile, has been downgraded from its peak as a Category 4 storm - which causes "catastrophic damage" - to a Category 1.

However, as hurricane-force winds began whipping North Carolina, federal emergency management officials warned that the hurricane remained a "very dangerous storm" capable of wreaking havoc along a wide swathe of the coast.

"Just because the wind speed came down ... please do not let your guard down," said Brock Long, the administrator of the Federal Emergency Management Agency (FEMA).

Warning of storm surges of up to 3.6m, he urged residents to take the storm seriously regardless of the category, saying "this is all about the water, anyway".

Screaming winds yesterday bent trees toward and rain flew sideways as Florence's leading edge whipped the Carolina coast, beginning an onslaught that could last for days, leaving a wide area under water from both heavy downpours and rising seas.

"The worst of the storm is not yet here but these are early warnings of the days to come," said North Carolina Governor Roy Cooper. "Surviving this storm will be a test of endurance, teamwork, common sense and patience."

About 10 million people could be affected by the storm and more than 1 million had been ordered to evacuate the coasts of the Carolinas and Virginia.

At least 12,000 people had taken refuge in 126 emergency shelters, Cooper said, with more facilities being opened.

The National Hurricane Centre warned that the threat of tornadoes was increasing as Florence neared shore and South Carolina Governor Henry McMaster said the heavy rains could trigger landslides in the western part of his state.

National Hurricane Centre director Ken Graham said on Facebook that the storm surges could push in as far as 3km.

Heavy rains were forecast to extend into the Appalachian Mountains, affecting parts of Alabama, Tennessee, Kentucky and West Virginia.

Officials said some 1.7 million people in the Carolinas and Virginia were warned to evacuate, but it's unclear how many did. Some residents rejected the calls.

Near the beach in Wilmington, North Carolina, a Waffle House restaurant, part of a chain with a reputation for staying open during disasters, had no plans to close, even if power was lost, and there were lines to get in last night.

Will Epperson, a 36-year-old golf course assistant superintendent, said he and his wife had planned to ride out the storm at their home in Hampstead, North Carolina, but then reconsidered. Instead, they drove 240km inland to his mother's house in Durham. "I've never been one to leave for a storm but this one kind of had me spooked," Epperson said.

- AP, news.com.au