The Bahamas is experiencing its strongest hurricane in modern times as Hurricane Dorian makes landfall in the islands.
The storm was upgraded to category five - the highest on the scale and rarely seen before.
Hotels have been boarded up and families have already evacuated their homes.
The National Hurricane Center (NHC) said "catastrophic conditions" were being experienced on the Abacos Islands - a group in the country's north.
Dorian's maximum sustained winds have increased to 185mph, according to the NHC, up from earlier readings of 160mph.
Warnings from the NHC tell of gusts up to 220mph in the path of the storm.
In the US, storm surge warnings and hurricane watches have been issued for a 154 mile (247 km) stretch of US coastline running north from Miami.
A storm surge warning means threat to life and a hurricane watch means hurricane conditions are expected within the next 48 hours.
Palm Beach county in Florida has ordered evacuations for low lying areas and those living in mobile homes.
At a press conference at the Federal Emergency Management Agency (FEMA) headquarters in Washington DC, President Donald Trump told residents of Florida, Georgia and North and South Carolina to listen to all the warnings and evacuation notices from local authorities.
He said Hurricane Dorian was "one of the largest we've ever seen, with effects seen hundreds of miles from the storm and long before it potentially makes landfall".
"We expect much of the eastern seaboard will be ultimately impacted - some of it very severely," he said.
The president added that his government was coordinating with local authorities and was ready to assist in rescue and recovery operations, should they be needed.
He praised the courage of the American people, by saying they are "strong, determined and resilient" and that they will support each other.
Dorian is moving west at 8mph, and its slow speed means it is expected to sit over the Bahamas for 24 hours before moving towards the US.
As well as extreme winds, 15-20 inches (38-50 cm) of rain is due to fall, with some areas receiving 25 inches (63.5 cm), according to the NHC's director Ken Graham. Waves could reach 15ft (4.5m) high, he added.
The NHC describes the expected extreme weather as "life-threatening" and "devastating".
Its advice includes:
Prime Minister Hubert Minnis of the Bahamas has urged residents to evacuate areas most at risk, saying that "homes, houses, structures can be replaced".
He added: "Lives cannot be replaced."
A spokesperson for the government of the Bahamas said Dorian was expected to impact some 73,000 residents and 21,000 homes.
Tourists have been sent into shelters built in schools, churches and other suitable buildings as people in homes and hotels have been evacuated.
Florida's governor Ron DeSantis has asked the public to remain vigilant, warning that dangerous storm surges are forecast for the region.
"As you're looking at these forecasts, a bump in one direction or the other could have really significant ramifications in terms of impact," the governor said at a news conference.