"If you are in one of the evacuation zones, you need to leave now".
The storm was heading for the coast of North and SC but heavy rain was also expected in Virginia to the north and Georgia to the south.
The storm is forecast to make landfall near Wrightsville Beach Friday morning before daybreak.
At the White House, President Donald Trump both touted the government's readiness and urged people to get out of the way of Florence.
The National Hurricane Center (NHC) said: "Florence is still forecast to be an extremely unsafe major hurricane when it nears the USA coast late Thursday and Friday". President Donald Trump declared states of emergency for North and SC and Virginia, opening the way for federal aid.
Faced with new forecasts that showed a more southerly threat, Georgia's governor joined his counterparts in Virginia and North and SC in declaring a state of emergency, and some residents who had thought they were safely out of range boarded up their homes.
However, there is also a risk of inland flooding as parts of North Carolina could receive 40 inches (1m) of rainfall.
"This is going to be a Mike Tyson punch to the Carolina coast", said Jeff Byard, associate administrator for response and recovery at the Federal Emergency Management Agency. "There's nobody left to work". "You can't stop Mother Nature". "I want to get them as far away as possible". The hurricane center said Florence will approach the coast Friday and linger for a while before rolling ashore.
More than 300,000 people have already evacuated from the coast, McMaster said, and traffic on the highway was moving steadily on Wednesday.
In Virginia, 245,000 coastal residents were ordered to evacuate.
Helene was weakening, however, and posed no danger to land, the NHC said, while Isaac could bring heavy rain to Martinique, Dominica and Guadeloupe.