Millions of Americans evacuated coastal areas of three states in the south-eastern United States ahead of the arrival of Hurricane Matthew on Friday, a powerful storm that ravaged the Caribbean earlier in the week, killing at least 283 in Haiti.

The governors of Florida, Georgia and South Carolina have ordered people living near the coast to leave their homes and made the flow of traffic on some major highways outbound only to facilitate the exodus.

"This storm will kill you," Florida Governor Rick Scott said at a press conference, calling on residents along the Florida peninsula's east coast to evacuate. "Don't take a chance," Scott said later on CNN. "Get out now."

As of 2 am Friday (0600 GMT), Matthew had weakened slightly to a Category 3 hurricane with maximum sustained winds of 195 kilometres per hour (kph), the Miami-based National Hurricane Centre said.

It was moving north-west at a speed of 22 kph, with the centre of the storm located 80 kilometres south-east of Cape Canaveral, Florida.

The eye of the storm had earlier passed over the western end of Grand Bahama Island after crossing over Freeport on a path toward the east coast of Florida. The hurricane centre said Matthew could cause "potentially disastrous impacts" in the south-eastern US state.

Scott ordered millions of people along the east coast of the state to evacuate, and warned that the hurricane was likely to cause power outages that could last for days.

Pentagon spokesman Peter Cook said on Twitter about 4,500 National Guard troops had been mobilized by their governors to support evacuation efforts and help people find shelter, food and water.

White House spokesman Josh Earnest warned that the storm was likely to be the largest and most powerful hurricane to hit the United States in a decade.

US President Barack Obama declared a state of emergency for much of Florida and all of South Carolina, freeing up federal resources to respond to the storm.

Matthew had killed at least 283 people in the worst affected area of Haiti, Department Sud, alone, Radio Television Caraibes reported citing local officials.

Other reports have put the death toll at well over 300.

Areas in Department Sud, in the south-west of the country, have been cut off from the rest of the country. Houses have been destroyed, trees knocked over and streets flooded.

"The situation is catastrophic," interim Haitian President Jocelerme Privert said.

More than 21,000 Haitians sought emergency shelter after the storm hit the impoverished Caribbean nation on Tuesday with winds of 230 kph. An estimated 350,000 people were in need of humanitarian assistance.

The US sent military helicopters and other aid to Haiti, and UN agencies were working to provide shelter, water and food, the UN said.

In the US, Georgia governor Nathan Deal ordered the mandatory evacuation of six coastal counties.

Elise Williams, a 28-year-old kindergarten teacher, was among those who chose to leave their homes in coastal Georgia along with her husband and 11-month-old daughter.

The family said they had made the decision to leave because they didn't want to risk being stuck in a dangerous storm with a baby or losing power for days. They left their home in Savannah at 6 am (1000 GMT) to avoid the rush of traffic on the drive to family members inland.

A "surprising number" of people however were choosing to stay in the historic Georgia city, Williams told dpa. "Some just think they can ride it out. Some think it won't be as bad as predicted."

The affected area of Florida includes Kennedy Space Centre, which closed on Thursday as employees worked to clear any debris that could cause damage to infrastructure, NASA said.

DisneyWorld in Orlando was to be closed through Friday, only the fourth time in its history that the amusement park has closed its gates, and the airport at Fort Lauderdale was also closed.

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