Media playback is unsupported on your device Media caption People have left homes and taken precautions ahead of the hurricane

US East Coast residents are running out of time to flee before Hurricane Florence hits the region as soon as Thursday evening, officials warn.

The storm was downgraded to category three with maximum sustained winds of 120mph (195km/h), but officials say it is still "extremely dangerous".

Up to 1.7 million people have been ordered to evacuate across South Carolina, North Carolina and Virginia.

Four South Carolina motorways have been diverted one-way to speed the exodus.

On Wednesday, Georgia declared a state of emergency, following the Carolinas, Virginia, Maryland and Washington DC.

A National Weather Service forecaster said: "This will likely be the storm of a lifetime for portions of the Carolina coast.

"And that's saying a lot given the impacts we've seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew.

"I can't emphasise enough the potential for unbelievable damage from wind, storm surge and inland flooding with this storm."

Media playback is unsupported on your device Media caption Why do people ignore hurricane warnings?

Jeff Byard, of the Federal Emergency Management Agency, said: "This is going to be a Mike Tyson punch to the Carolina coast."

North Carolina Governor Roy Cooper warned that "disaster is at the doorstep", and "tens of thousands" of buildings may be flooded.

Waves 83ft (25m) tall were recorded at sea on Wednesday morning.

But while many coastal residents have complied with mandatory evacuation orders, others are boarding up their homes and vowing to ride out the storm.

Image copyright Getty Images Image caption A holidaymaker from Ohio says he plans to ride out the storm from his rental home in North Carolina's Outer Banks

Image copyright Getty Images

In other developments:

Florence could wreak more than $170bn (£130bn) of havoc and damage nearly 759,000 homes and businesses, says analytics firm CoreLogic

North Carolina farms are moving livestock to safety. In 1999, Hurricane Floyd left hundreds of thousands of dead hogs and chickens floating in floodwaters in the state

Nearly 1,000 prisoners in South Carolina will not be moved from their cells, despite a mandatory evacuation order in the area. "In the past, it's been safer to leave them there," said a state department of corrections spokesman

There are fears for wild horse herds in North Carolina's Outer Banks, but experts say these equines can sense bad weather coming and head to higher ground

In a video posted to his Twitter account on Wednesday, US President Donald Trump warned residents in Florence's bullseye to heed official warnings.

"Get out of its way, don't play games with it," said Mr Trump. "It's a big one, maybe as big as they've seen, and tremendous amounts of water."

"Bad things can happen when you are talking about a storm this size. It's called Mother Nature. You never know, but we know. We love you all, we want you safe."

European astronaut Alexander Gerst shared images of Florence's eye wall from space, calling the storm a "no-kidding nightmare".

What makes Florence so dangerous?

Forecasters say the storm poses such a threat because it is expected to slow down and hover for nearly two days over the Carolina coast, before dipping south towards Georgia.

It is forecast to bring 20-40in (50-100cm) of rain and life-threatening storm surges of up to 13ft. As of Wednesday evening, officials say these surges are now "highly likely" as Florence churns closer.

The National Weather Service also predicts "catastrophic flash flooding and prolonged significant river flooding" in parts of the Carolinas and Appalachians.

Hurricane force winds will emanate up to 70 miles from the centre of the storm, say meteorologists.

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National Hurricane Center Director Ken Graham warned that waterways up to 40 miles inland may flood.

Mr Graham said the Pamlico and Neuse rivers in North Carolina will see their flows "reversed" as storm surges push water back inland.

He added that half of fatalities during hurricanes are caused by storm surges, and another quarter of deaths are due to inland rains and flooding.

Image copyright Getty Images Image caption The barrier islands of the Carolina coastline could be prone to flooding

Is global warming to blame?

The relationship between climate change and hurricanes is a complex one.

Warmer seas power hurricanes. So as the temperature of ocean water goes up, we might expect the intensity of hurricanes to increase in future.

A hotter atmosphere can also hold more water, so this should allow hurricanes to dump more water on affected areas.

But there are so many factors that contribute to these rare events, it has been difficult to tease out clear trends from the data.

Image copyright EPA Image caption Chuck Ledford (L), watches Looney-Tunes with his daughter Misty as they evacuate in Wilmington, North Carolina

Image copyright Getty Images Image caption Navy ships that are under repair and cannot go to sea are secured to the Norfolk, Virginia, port with heavy moorings

Hurricanes A guide to the world's deadliest storms Hurricanes are violent storms that can bring devastation to coastal areas, threatening lives, homes and businesses. Hurricanes develop from thunderstorms, fuelled by warm, moist air as they cross sub-tropical waters.

Warm air rises into the storm. Air swirls in to fill the low pressure in the storm, sucking air in and upwards, reinforcing the low pressure. The storm rotates due to the spin of the earth and energy from the warm ocean increases wind speeds as it builds. When winds reach 119km/h (74mph), it is known as a hurricane - in the Atlantic and Eastern Pacific - or a typhoon in the Western Pacific. "Everybody has a plan until they get punched in the face. Well, we're about to get punched in the face."

Florida Mayor Bob Buckhorn, ahead of Hurricane Irma (2017) The central eye of calmer weather is surrounded by a wall of rainstorms.

This eyewall has the fastest winds below it and violent currents of air rising through it. A mound of water piles up below the eye which is unleashed as the storm reaches land.

These storm surges can cause more damage from flooding than the winds. "Urgent warning about the rapid rise of water on the SW FL coast with the passage of #Irma's eye. MOVE AWAY FROM THE WATER!"

Tweet from the National Hurricane Center The size of hurricanes is mainly measured by the Saffir-Simpson scale - other scales are used in Asia Pacific and Australia. Winds 119-153km/h

Some minor flooding, little structural damage.

Storm surge +1.2m-1.5m Winds 154-177km/h

Roofs and trees could be damaged.

Storm surge +1.8m-2.4m Winds 178-208km/h

Houses suffer damage, severe flooding

Storm surge +2.7m-3.7m Hurricane Sandy (2012) caused $71bn damage in the Caribbean and New York Winds 209-251km/h

Some roofs destroyed and major structural damage to houses.

Storm surge +4m-5.5m Hurricane Ike (2008) hit Caribbean islands and Louisiana and was blamed for at least 195 deaths Winds 252km/h+

Serious damage to buildings, severe flooding further inland.

Storm surge +5.5m Hurricane Irma (2017) caused devastation in Caribbean islands, leaving thousands homeless "For everyone thinking they can ride this storm out, I have news for you: that will be one of the biggest mistakes you can make in your life."

Mayor of New Orleans Ray Nagin ahead of Hurricane Gustav, 2008 Click arrow to proceed Loading ... Swipe to progress Go back to start

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