At least 842 people have been killed by Hurricane Matthew in Haiti, local officials said, as the death toll continues to climb.
Recovery efforts are still underway in the Caribbean island state, where entire villages were crushed by winds and storm surges.
The hurricane is now hitting the Florida coast, where around two million people have been told to evacuate.
Thousands of residents in Florida have been spared the worst of hurricane Matthew as the storm's centre hovered just offshore.
Nearly 600,000 homes and business were left without power as high winds of up to 107 mph thrashed the area around Daytona beach, whipping up storm surges, toppling trees and ripping off roofs.
However many of the most populous areas were spared as the storm swerved away from shore, avoiding many of the state's most heavily-populated areas.
President Obama said the new areas of concern are now Jacksonville in northern Florida and the neighbouring state of Georgia as Hurricane Matthew moves north.
Florida governor Rick Scott has also reiterated warnings to thousands of residents ordered to evacuate, saying: "Remember, it could be the worst of it is yet to come."
At least 572 people are now known to have died in Hurricane Matthew in Haiti, local officials said.
The death toll is likely to continue to rise as rescue teams continue to search devastated areas after the hurricane blasted over the island state.
Haiti has suffered the brunt of the ferocious storm so far, though the hurricane is now headed for the south coast of the US.
Thousands more than been left homeless, with aid agencies warning that up to 500,000 children are at risk of disease as families are left without shelter, food and clean water.
More than 22,000 Florida residents are in shelters and 600,000 state homes are without power as Hurricane Matthew batters the state.
Florida's governor Rick Scott urged people to remain vigilant, warning that the storm "is not over".
Officials are still assessing the damage after Category 3 Matthew made landfall in the early hours of Friday on Florida's east coast, while there is potential for significant flooding in Jacksonville.
As of yet no fatalities have been reported in the immediate aftermath.
ITV News Washington Producer Jamie Roberton tweets:
- Video report by ITV News Correspondent Rebecca Barry
Unicef Haiti communications chief Cornelia Walther has claimed the situation in the country is "worse than we expected".
Aid agencies have warned almost 500,000 children are at risk of disease in Haiti after a devastating hurricane battered the country, causing extensive damage.
Speaking to ITV News from Port-au-Prince on Friday, Walther said: "From all the information we get on the ground, the situation is worse than we expected.
"It's a very serious situation and right now we're assessing what the immediate needs are. What is certain is that access to sanitation is a major priority to prevent epidemics.
"[The next priority] is to identify the most vulnerable - including separated children. Right now one of the key concerns is the access to clean water."
She added: "We hear that 70-80% of houses have been damaged and schools have been closed or flooded - or are used as emergency shelter. The situation is very very worrying."
The death toll from Hurricane Matthew in Haiti has risen to 478.
Haitian officials had feared the toll could rise, with rescue crews now reaching remote areas of the country who were previously cut off by the storm.
An estimated 61,500 people remain in emergency shelters after the country suffered its first Category 4 storm in more than 50 years.
Two million people in Florida have been warned to flee inland as winds of up to 120mph and driving rain batters the south-east coast of the US.
Florida governor Rick Scott warned: "This storm's a monster. I'm going to pray for everybody's safety. The storm has already killed people. We should expect the same impact in Florida."
The number of homes and businesses without power jumped to 300,000 by Friday morning.
President Barack Obama declared a state of emergency for Florida, Georgia and South Carolina - who are expected to be hit this weekend - freeing up federal money and personnel to protect lives and property.
Georgia's governor Nathan Deal has ordered an evacuation of its entire east coast - affecting more than 500,000 people.
Meanwhile Amtrack have suspended all train services between Miami and Florida, and cruise liners have been instructed to avoid the area.
Thousands of people have taken shelter in schools, with inland hotels in Charlotte and North Carolina reporting brisk business.
Pilus Enor, the Mayor of Camp Perrin, Haiti, has said the devastation caused by Hurricane Matthew can be seen "everywhere".
He told reporters: "Devastation is everywhere. Every house has lost its roof. All the plantations have been destroyed. This is the first time we see something like this".
On Friday, the International Red Cross announced an emergency appeal for £5.6m to provide medical aid, shelter, sanitation assistance and water to 50,000 people in south-western Haiti.
Unicef said they need £4m to support children's immediate needs in the country.
Rescue teams have begun to arrive in the remote areas of the country and authorities are fearing the number of deaths to rise, with bodies starting to appear as waters recede.
Officials are especially concerned for the area of Grand-Anse, where they believe the death toll and damage is highest.
Health officials in Haiti fear a surge in cholera cases in the wake of Hurricane Matthew.
The storm has already killed more than 300 people in the Caribbean country and destroyed thousands homes.
Matthew also damaged Haiti's water supplies and sanitation systems, sparking fears of an outbreak in the potentially fatal bacterial infection - caused by consuming contaminated water.
Following the 2010 Haiti earthquake, UN peacekeepers inadvertently introduced cholera to the country, killing at least 9,000 people and infected hundreds of thousands more.
Flights between the UK and Florida have been cancelled or delayed by Hurricane Matthew.
Flights in and out of Orlando have been suspended and the Foreign Office has urged people in the area to monitor weather reports and follow the advice of the local authorities.
Virgin Atlantic is warning passengers of overnight delays affecting a number of flights on Friday and Saturday from Manchester, Glasgow and Gatwick.
Thomas Cook Airlines has delayed Glasgow and Manchester flights to and from Orlando on Friday by 24 hours due to the weather.
In updated travel advice, the Foreign Office said: "If you're in an affected area, you should call family members and friends in the UK to let them know your plans before and after the hurricane."