More than two million people on the US southeast coast faced an urgent evacuation order Thursday as monstrous Hurricane Matthew bore down on track for a direct hit in Florida, braced for waves as high as two-story buildings.

Up and down the coast, highways clogged as people fled inland to escape the storm, which killed at least 27 people as it blasted its way through the Caribbean.

Haiti remained essentially cut in half.

As US gas stations ran dry, frantic shoppers flocked to stores for essentials. They snapped up batteries, transistor radios, bread, canned goods, bottled water and other items in preparation for what Florida Governor Rick Scott described as a devastating, killer storm, with winds howling at up to 150 miles per hour (240 kph).

"Evacuate, evacuate, evacuate," Scott told a news conference. "Time is running out."

Around 1.5 million coastal dwellers are under evacuation order in his state. More than a million others in South Carolina and other coastal states were also told to escape the path of the Category Three storm, forecast to go back up to level four -- the strength at which it hit Haiti Tuesday when it first made landfall.

Mandatory evacuations were also ordered in six coastal counties in Georgia.

Miami International Airport cancelled 90 percent of its incoming and outgoing flights on Thursday.

The National Hurricane Center called Matthew the strongest in the region in decades.

It said waves whipped up by the hurricane could be as high as 18 feet (5.5 meters). Debris tossed into the air by the storm will be capable of blasting through buildings and cars, the NHC said in a bulletin.

It said the storm is predicted to be very near or over the east central coast of Florida Thursday night or early Friday.

Scott said the forecast is for storm surges of five to nine feet (1.5 to 2.7 meters), not counting the waves on top of that.

"Stop and think about that," he said. "Waves will be crashing on your roof if you're right close to where the storm surge is happening and you're close to where the waves are."

He said power outages, possibly lengthy, are a near certainty.

'It's pretty bad'

Amid the massive flight, officials warned a worrying number of people were not heeding the evacuation order.

In South Carolina's coastal Charleston and Beaufort counties, Governor Nikki Haley said 175,000 people had evacuated as of Thursday morning -- out of 250,000 who were told to leave.

"That is not enough, we need to have more people evacuating," she told a news conference. "If you are still sitting at home, if you have not evacuated, gas stations are getting ready to close, your pharmacies are getting ready to close, everything is going to leave."

Among the holdouts in Florida was Judy Ruscino, 74, who said she and her husband hoped to ride out the storm in their home in Daytona Beach, in Florida's north.

"It's a little bit scary. I know it's pretty bad but we have the sand, we bought food, the garage door is storm proof," said Ruscino.

As of Thursday morning the storm was swirling near the northwest Bahamas.

As Matthew barreled northwest, Haiti and Cuba continued the grim task of assessing damage and fatalities.

Matthew's provisional death toll stood at 27 -- 23 in Haiti and four in the Dominican Republic -- but this looked certain to climb.

Civil protection spokesman Edgar Celestin said the figures do not include data from the department of Grande Anse, which was in the eye of the storm and has been cut off from communication.

Haiti's presidential election, scheduled for Sunday, has been postponed.

Interim president Jocelerme Privert, who surveyed damage to southern Haiti Wednesday in a US Coast Guard plane, called the situation catastrophic.

Matthew has knocked out a key bridge providing access to the country's south where the storm made landfall.

The severe flooding has also sparked a resurgence of cholera, with eight cases already reported, officials said.

The United Nations office for coordinating humanitarian affairs said half of Haiti's population of 11 million was expected to be affected.

At least 350,000 people in Haiti, where thousands have lived in tents since the massive earthquake in 2010, need immediate assistance, UN Secretary General Ban Ki-moon's office said.

The US military said Wednesday it was sending helicopters, an aircraft carrier and troops to assist the aid effort in Haiti -- which had not been hit by a Category Four storm in 52 years.

In Cuba, where some 1.3 million people were evacuated, there were no reported fatalities but four cities in the east were cut off because roads were blocked by large chunks of rock hurled by the storm.

Officials reported flooding and waves up to 16 feet (five meters) in eastern coastal villages and Baracoa, the first Spanish settlement in Cuba, was decimated.

"There's nothing left of Baracoa. Just debris and remains," said resident Quirenia Perez, 35, speaking to AFP by cell phone after losing her roof, electricity and landline in the storm.