Forecasters said their hurricane hunter aircraft found Florence strengthening as it churned about 720 miles (1,160 kilometers) southeast of Bermuda.

The states of North and South Carolina and Virginia -- all potentially in Florence's crosshairs -- have already issued emergency declarations to help speed preparations.

"This storm is too powerful and its path is too uncertain to take any chances," South Carolina Governor Henry McMaster said in issuing his state's emergency declaration.

The US navy has ordered ships at its Hampton Roads, Virginia base to put to sea, saying "the forecasted destructive winds and tidal surge are too great to keep the ships in port."

The NHC said the vast storm had maximum sustained winds of 85 miles per hour, reaching up to 125 miles from its center, as it tracked west-northwestward across the western Atlantic.

The storm was producing large swells expected to reach from the northern Caribbean to the southern coasts of Canada's Maritime provinces.

These swells, already affecting Bermuda, "are likely to cause life-threatening surf and rip current conditions," the NHC warned.

Florence was the first Category 4 hurricane of the Atlantic season, but weakened as it passed through cooler waters. It is now entering a warmer zone that could lead it to gather strength, meteorologists said.

"Interests along the US East Coast, particularly Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place and follow any advice given by local officials," the NHC said.

At this statistical height of the Atlantic hurricane season, Florence was being trailed on east-to-west paths by Hurricane Helene near the Cabo Verde islands, and Tropical Storm Isaac.

Isaac was on a path that could take it to the Leeward or Windward islands, many of which are still struggling to recover from Hurricane Maria.

Maria -- which killed at least 3,057 people, most in Puerto Rico -- is believed to be the third costliest tropical cyclone on record.

The US government has been criticized for its response to Maria in Puerto Rico, a US commonwealth in the Caribbean.