Herve Fourcand, a senator for the Sud department, which felt the full force of the impact, said he had so far recorded 400 deaths with several localities still inaccessible.
The civil protection agency for the Sud told AFP it has recorded 315 deaths, with at least four towns still to report back on local fatalities.
The western edge of Hurricane Matthew's eyewall is brushing the US state of Florida's coast at Cape Canaveral.
A Brevard County Emergency Operations spokesman said early on Friday that more than 100,000 people in the area had lost power.
The National Hurricane Center reported a wind gust of 100 mph (160 kph) occurring in Cape Canaveral early on Friday.
The Category 3 storm is centered about 25 miles (40 kilometres) east of Cape Canaveral and is moving north-northwest at 14 mph (22 kph).More than 360,000 in Florida were without power early on Friday.
Matthew left more than 400 dead in its wake across the Caribbean and is now battering Florida's east coast with high winds and rain
12