Hurricane Matthew has made landfall in the south-east US state of South Carolina, according to the National Hurricane Centre, reporting maximum sustained winds of 120 kph and and expecting serious inland flooding.
The storm was earlier downgraded to a Category 1 storm and continues to move north-east at a speed of 19 kph, with storm surges bringing up to a 1.5-metre rise in water levels.
Deadly flooding over the next 36 hours could hit the states of Georgia and South Carolina, th NHC warned, with local weather services already reporting flooding of up to 3 metres in some areas.
As of Saturday, there have been at least five deaths reported in the United States, with over one million people in Florida, George and South Carolina without electricity overnight.
Matthew weakened late on Friday to a category 2 storm with maximum sustained winds of 175 kilometres per hour, after leaving a path of devastation across Haiti as a category-4 storm.
The CARE aid organization, citing Interior Ministry figures, put the death toll at 478, with international media citing even higher figures.
At least 283 people were reported dead in Haiti's hard-hit Sud department, radio station Metropole reported, citing the local civil protection agency.
In the Haitian city of Jeremie, 80 percent of the houses were destroyed or damaged, CARE activist Holly Frew said Friday, adding that the number of victims was expected to rise, as some remote regions remained largely cut off from the rest of the country.
The EU has pledged an additional 1.5 million euros of emergency aid to victims in Haiti and sent a team to affected areas to help administer water and aid.