Hurricane Florence is expected to become a dangerous "major hurricane" by late Monday as it heads toward the US East Coast, the National Hurricane Centre said, as states of emergency were declared in preparation for the storm.
The center of Florence was located about 1 100km southeast of Bermuda, the NHC in its 03:00 GMT on Monday advisory.
Florence had maximum sustained winds of 128km/h, making it a Category 1 storm on the five-level Saffir-Simpson hurricane scale.
The NHC warned that Florence "is forecast to rapidly strengthen to a major hurricane by Monday night, and is expected to remain an extremely dangerous major hurricane through Thursday".
The storm is moving towards the west at 11km/h, and is forecast to drench a large swath of the US East Coast running from northern Florida to New Jersey.
On its current track Florence is expected to slam the Carolinas and Virginia the hardest - and all three states have issued emergency declarations to speed preparations.
Virginia Governor Ralph Northam's office described Florence as possibly the state's "most significant hurricane event in decades", warning of "catastrophic inland flooding, high winds and possible widespread power outages".
It added: "The largest threat to life from hurricanes is not the high winds. Flooding is the deadliest result of these storms."
The US navy has ordered ships at its major base in 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".
Two more hurricanes
North Carolina Governor Roy Cooper's office said that Florence is already being felt along the state's coast, with large sea swells resulting in life-threatening rip currents and surf.
"Everyone in North Carolina needs to keep a close eye on Florence and take steps now to get ready for impacts later this week," Cooper said.
The 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.
Florence was producing large swells expected to reach from the northern Caribbean to the southern coasts of Canada's Maritime provinces.
At this statistical height of the Atlantic hurricane season, Florence was being trailed on east-to-west paths by two hurricanes, Helene and Issac.
Helene - currently just southeast of the Cabo Verde islands off the African coast - had winds of 136km/h, and was expected to turn northwest and then north into the open Atlantic by midweek, the NHC said.
Hurricane Isaac - which late Sunday became the fifth hurricane of the season - is heading west towards the Caribbean.
At 03:00 GMT Issac was about 2 100km east of the Windward Islands - a region still recovering from last year's powerful Hurricane Maria - with winds of 120km/h.
Issac is expected to gain strength in the next days, but then weaken by the middle of the week when it approaches the Caribbean.
Maria - which killed at least 3 057 people, most in Puerto Rico - is believed to be the third costliest tropical cyclone on record.