Panama City, Oct 10 (AFP) Hurricane Michael slammed into the Florida coast on Wednesday as the most powerful storm to hit the southern US state in more than a century as officials warned it could wreak "unimaginable devastation".

Michael made landfall as a monstrous Category 4 storm near Mexico Beach, a town about 20 miles (32 kilometers) southeast of Panama City, around 1:00 pm Eastern time (1700 GMT), the National Hurricane Center said.

As the eye of the hurricane came ashore, winds of up to 155 miles per hour (250 kilometres per hour) and driving rain pounded beachfront communities on the Florida Panhandle, the finger-shaped strip of land along the Gulf of Mexico.

"Hurricane Michael is forecast to be the most destructive storm to hit the Florida Panhandle in a century," Governor Rick Scott said.

Briefing President Donald Trump at the White House, top emergency management official Brock Long said Michael was the most intense hurricane to strike the area since 1851.

"Along our coast, communities are going to see unimaginable devastation," Governor Scott said.

"The National Hurricane Center is expecting storm surge to be between nine and 13 feet (2.7-3.6 meters)," he added. "Water will come miles in shore and could easily rise over the roofs of houses." Hundreds of thousands of people were ordered to evacuate their homes and the governor told residents that if they have not already done so it was now too late.

"The time to evacuate the coastal areas has come and gone... Hunker down, and be careful," he said. "Don't go out in the middle of this. You are not going to survive it. It's deadly." "This is, unfortunately, a historical and incredibly dangerous and life-threatening situation," said Ken Graham, director of the Miami-based NHC. "It's going to be incredibly catastrophic."

Long, the head of the Federal Emergency Management Agency (FEMA), said many Florida buildings were not built to withstand a storm above the strength of a Category 3 hurricane on the five-level Saffir-Simpson Hurricane Wind Scale.

As it came ashore, Michael was just shy of a Category 5 -- defined as a storm packing wind speeds of 157 mph or above.

"We're going to see a lot of wind damage," Long said, and some residents could expect not to have power restored for weeks.

Speaking before the hurricane made landfall, Long said "this is the final call for anybody that needs to get out.

"Those who stick around to experience storm surge don't typically live to tell about it unfortunately," he said.

Residents of the neighbouring state of Georgia should also expect to be heavily impacted by the storm, the FEMA head said. "Citizens in Georgia need to wake up and pay attention," Long said.

Mike Thomas, the mayor of Panama City Beach, a resort west of Panama City, said he expected there would be casualties and that emergency personnel would not go out when winds get over 50 miles per hour (80 kph).

"It's going to be horrible," Thomas told CNN. "We're going to get some people hurt." An estimated 375,000 people in more than 20 counties were ordered or advised to evacuate.

The National Weather Service office in the state capital Tallahassee issued a dramatic appeal for people to comply with evacuation orders.

"Hurricane Michael is an unprecedented event and cannot be compared to any of our previous events. Do not risk your life, leave NOW if you were told to do so," it said.

The NWS said it had found no record of any previous Category 4 hurricanes that made landfall in the Panhandle or the "Big Bend" coastal region.

"This situation has NEVER happened before," it said on Twitter.

Trump issued an emergency declaration for Florida, freeing up federal funds for relief operations and providing the assistance of FEMA, which has more than 3,000 people on the ground.

State officials issued disaster declarations in Alabama and Georgia and the storm is also expected to bring heavy rainfall to North and South Carolina.

The Carolinas are still recovering from Hurricane Florence, which left dozens dead and is estimated to have caused billions of dollars in damage last month.

It made landfall on the coast as a Category 1 hurricane on September 14 and drenched some parts of the state with 40 inches (101 centimeters) of rain.

Last year saw a string of catastrophic storms batter the western Atlantic -- including Irma, Maria and Harvey, which caused a record-equaling $125 billion in damage when it flooded the Houston metropolitan area.

Scientists have long warned that global warming will make storms more destructive, and some say the evidence for this may already be visible. (AFP)

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