US president Donald Trump cancelled a trip to Poland as Hurricane Dorian headed towards Florida, strengthening on Thursday to a Category 2 storm.
Trump, who had been scheduled to attend World War II anniversary commemorations this weekend, said he would focus instead on preparations for the approaching storm.
"Our highest priority is the safety and security of the people in the path of the hurricane," he told reporters in the White House Rose Garden.
Vice president Mike Pence would go to Poland in his place, Trump said.
He called his Polish counterpart Andrzej Duda to apologize for the cancelation, saying the storm could be "catastrophic," the president's chief of staff Krzysztof Szczerski said, according to the PAP agency.
Dorian's maximum sustained winds reached 105 miles per hour (165 kilometers per hour), making it a Category 2 storm on a five-level scale, the National Hurricane Centre (NHC) said in an advisory.
Weather forecasters said Dorian could make landfall as a powerful Category 4 hurricane with the potential to cause life-threatening storm surges.
Florida's governor 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 major hurricane.
"All Floridians really need to monitor Hurricane Dorian and make the necessary preparations," Governor Ron DeSantis said. "Have your plan and make those preparations right now."
- Catastrophic impacts -
Georgia -- another southeastern state potentially 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 full of 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 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 centimetres) 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 the hurricane and the months of disruption that followed.