Washington (AFP) - Hurricane Irma will have a "truly devastating" impact when it slams into southern coastal areas of the United States, the head of the US emergency agency said on Thursday.
Federal Emergency Management Agency (FEMA) chief Brock Long said people in Florida and other states must heed evacuation orders as the Category Five hurricane surges towards the US after causing death and destruction in the Caribbean.
The FEMA chief said Irma would be only the fourth Category Five hurricane to hit the United States since 19851 and the first since Hurricane Andrew in 1992.
"Bottom line is the majority of people along the coast have never experienced a major hurricane like this. It will be truly devastating," he told CNN.
While mandatory evacuation orders have so far only been issued in parts of Florida, he expected similar orders for Georgia, South Carolina and North Carolina within the next 48 hours, Long said.
"The entire southeastern United States better wake up and pay attention," he added.
Evacuation orders have already been issued in parts of Miami, including Miami Beach, one of the country's most popular tourist areas.
The mayor of Miami Beach Philip Levine has warned that Irma could have an impact of "nuclear" proportions as he urged everyone to get out of its path as soon as possible.
"I strongly urge them to please leave Miami Beach. You have friends, you have family -- go visit them," he told Miami's CBS4 News station on Wednesday night.
"This is a nuclear hurricane. They should leave the beach, they must leave the beach."
Long, whose agency is still busy with the aftermath of Hurricane Harvey, which hit Texas and Louisiana last month, said around 3,000 federal workers have been mobilized to deal with the emergency.
Three navy ships are also anchored off the coast of Puerto Rico "to support life-saving missions", said Long, although there have so far been no reports of major damage after Irma swept over the island on Wednesday night.