Dorian heading toward Florida on Friday was on track to become a major hurricane with a dangerous storm surge and potential for life-threatening flash floods.
Its maximum sustained winds have reached 105 miles per hour (165 kilometers per hour), making it a Category 2 storm on a five-level scale, the National Hurricane Center said.
It is expected to strengthen over the next few days as it approaches the Bahamas and then Florida over the weekend.
Weather forecasters said Dorian could make landfall on the US east coast as a powerful Category 4 hurricane with the potential to cause life-threatening storm surges.
President Donald Trump cancelled a weekend trip to Poland to focus on preparations for the storm.
Florida has declared a state of emergency, warning the millions of people who live up and down the eastern coast of the Sunshine State to prepare for a potentially huge hurricane.
- 'Catastrophic impacts' -
Georgia -- another southeastern state that could be in the storm's path -- followed suit for 12 counties.
Governor Brian Kemp said the hurricane "has the potential to produce catastrophic impacts to citizens" throughout the southeast coastal region of the United States.
Trump, who has properties including the Mar-a-Lago resort in Florida, a politically important state for his 2020 re-election bid, also warned Floridians to get ready.
"Be prepared... it will be a very big Hurricane, perhaps one of the biggest!" he said on Twitter.
The US Coast Guard said ocean-going commercial vessels should make plans to leave South Florida ports.
"The risk of devastating hurricane-force winds along the Florida east coast and peninsula late this weekend and early next week continues to increase," the NHC warned.
Miami's Ocean Drive, a normally-vibrant strip along the beach, was quiet, with some empty tables and fewer tourists than usual.
Grocery stores were packed with shoppers making last-minute purchases of water, food, propane canisters and other supplies. There were lines at some gasoline stations as drivers filled up their tanks.
One shopper, Magdalena Gomez from Argentina, was preparing for her first hurricane: "If they tell me to go buy water, I go buy water. I do everything they say."
A Rolling Stones concert in Miami originally scheduled for Saturday night was moved up by a day "due to the weather forecast," the band's Twitter account said.
- Puerto Rico spared -
"Strengthening is forecast during the next few days, and Dorian is expected to become a major hurricane on Friday, and remain an extremely dangerous hurricane through the weekend," the NHC said.
The US territory of Puerto Rico, still recovering from a powerful storm two years ago, was largely spared, but the NHC said Dorian could dump up to seven inches (18 centimeters) of rain on some parts of the Bahamas.
In Puerto Rico, the new governor, Wanda Vazquez, gave the all-clear, but there was no let-up in the political storm involving Trump and Carmen Yulin Cruz, the mayor of San Juan, the island's capital.
"Now that Dorian is going to the east coast let us hope that @realDonaldTrump sets aside his prejudice and racism & moves the federal response to efficiency," the mayor tweeted.
Trump declared a state of emergency in Puerto Rico ahead of Hurricane Dorian's anticipated arrival, authorizing federal assistance, but said that the island is "one of the most corrupt places on earth" and its political system is "broken."
Puerto Rico was devastated in 2017 by Hurricane Maria, a Category 4 storm.
A study accepted as valid by the government, which initially put the death toll at 64, estimated that nearly 3,000 people died as a result of that hurricane and the months of disruption that followed.