Thousands of Florida residents have been ordered to leave their homes as Hurricane Michael heads for the state's coast.
The storm has already killed 13 people in Central America as it battered Mexico's Yucatan Peninsula and western Cuba with rain and winds of up to 85 mph.
By the time it reaches Florida on Wednesday local time, it is expected to be a category three hurricane with winds of more than 100mph and storm surges of up to 12ft (3.7 metres).
This would make it the strongest storm to hit the Panhandle of the state since Hurricane Dennis in 2005.
Florida governor Rick Scott has declared a state of emergency in 35 counties, warning that the region must be "prepared for a direct hit with sustained hurricane force winds".
He described Michael as a "monstrous hurricane".
Some 1,250 National Guard soldiers are helping residents in coastal areas to find safety, and another 4,000 are on standby, Mr Scott said.
On the Panhandle, Escambia County Sheriff David Morgan warned people against staying in their homes, saying emergency services would not be able to help them.
He said: "If you decide to stay in your home and a tree falls on your house or the storm surge catches you and you're now calling for help, there's no one that can respond to help you."
Many schools, businesses and government buildings will be closed and energy companies have halted nearly a fifth of the oil production on the Gulf of Mexico, a part of the US which is responsible for 17% of daily US crude oil output.
Lines have formed at petrol stations and people have been stocking up on emergency food supplies.
Even political campaigning took a back seat for Tallahassee Mayor Andrew Gillum, Florida's Democratic nominee for governor, who instead helped residents fill sandbags.
He said: "Today is about life and safety.
"There's nothing between us and this storm but warm water and I think that's what terrifies us about the potential impacts."
Neighbouring Alabama is also expecting strong winds, heavy rain and power cuts as a result of the hurricane.
Michael is moving north at 12mph and, after hitting Florida, it is expected to move up the east coast on Wednesday and Thursday, into North and South Carolina.
The two states are still recovering from Hurricane Florence last month.
More from Alabama
Meanwhile, Tropical Storm Leslie is forecast to strengthen into a hurricane on Wednesday.
Leslie is around 1,090 miles west of the Azores, an archipelago composed of nine volcanic islands in the North Atlantic, with maximum sustained winds of 60mph.
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Sky News
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