The storm was the most powerful hurricane to hit the continental US in more than 50 years.
"This morning, Florida's Gulf Coast and Panhandle and the Big Bend are waking up to unimaginable destruction," state Governor Rick Scott said. "So many lives have been changed forever. So many families have lost everything. ... This hurricane was an absolute monster."
So far, at least two deaths have been attributed to the storm, but vast swaths of property were levelled.
More than 375,000 people along the Gulf Coast were given mandatory evacuation orders, but many people defied the calls. Some areas remain out of reach to authorities.
Brock Long, head of the Federal Emergency Management Agency, said Mexico Beach was "ground zero" for damage. The town of 1,200 people was largely leveled, as it was hit by 155 miles an hour (250 kilometers an hour) winds and a storm surge of 9 feet (2.7 meters) from the category 4 hurricane.
Long said there could still be people trapped in the area, known for its small coastal towns and wildlife reserves.
More than 900,000 homes and businesses in Florida, Alabama, Georgia and the Carolinas have been left without power. Several thousands National Guard troops, law enforcement officers and medical teams have been deployed in the recovery effort.
The storm has weakened but continues to travel over the Carolinas to the Atlantic Ocean, bringing with it heavy rains. The rains are causing flash flooding across parts of North Carolina and southern Virginia.
aw/ng (AP, Reuters, AFP)