Hurricane Dorian is slamming into the north-western Bahamas as a catastrophic category five storm with sustained winds of up to 180mph (285km/h).
The “extremely dangerous” storm is the strongest hurricane on modern record in the area, the US National Hurricane Center (NHC) says.
Grand Bahama residents have been evacuating from its predicted path.
A storm surge could be life-threatening and as high as 23ft (7m) in places, officials warn.
At 11:00 local time (15:00 GMT) the NHC reported “catastrophic conditions” in the Abaco islands in the northern Bahamas.
Dorian, moving westward at 7mph (11km/h), is expected to continue to move over Great Abaco Island and near, or over, Grand Bahama Island later on Sunday into Monday.
Authorities closed some airports in the outlying islands, but the main international airport remains open on Sunday.
After hitting the Bahamas, it should move closer to the Florida east coast late on Monday through Tuesday night local time.
US President Donald Trump said he was monitoring Dorian. He cancelled a planned trip to Poland, sending Vice-President Mike Pence in his place.
In a tweet on Sunday, Trump warned Dorian was expected to hit “much” harder than expected and urged residents in its path to be careful.
Hurricanes, which vary in strength from category one to five on the Saffir-Simpson scale, tend to get stronger as they move over warm waters like those off the coast of Florida.
Category five hurricanes, with sustained wind speeds of at least 157mph, are extremely rare, although four other storms – Michael, Maria, Irma and Matthew – reached this level in the last three years.
Dorian has also grown larger in size, with hurricane-force winds currently extending 45 miles (75km) from the storm’s centre.
Prime Minister Hubert Minnis announced an evacuation order for parts of Grand Bahama and the Abaco Islands, both in the north of the archipelago. All tourists were asked to leave the islands.
At a news conference, Mr Minnis begged residents to head for the main island to escape the “devastating, dangerous storm”.
“I want you to remember: homes, houses, structures can be replaced. Lives cannot be replaced,” he said, adding that 73,000 people and 21,000 homes were in danger.