The most powerful hurricane to threaten the Atlantic coast in more than a decade, Irma has swelled into a monster force with maximum sustained winds near 185 mph (297 km), according to the National Hurricane Center.
Irma’s eye is expected to pass about 50 mph (80 km) from Puerto Rico on Thursday morning, and forecasters say the storm will retain its power for at least the next five days.
Irma is estimated to hit Florida at around 2 am (6 am GMT) on Sunday morning.
As dire warnings mounted, schools and offices began to shut down and authorities ordered evacuations with more to follow.
A state of emergency has been declared throughout Florida and the US territory of Puerto Rico and the US Virgin Islands.
The category 5 hurricane has been named the most powerful ever recorded over the Atlantic Ocean.
It smashed the Caribbean coast and is forecast to devastate the South Eastern region of the US.
Officials warned locals to seek protection from Irma’s “onslaught”.
On Tuesday, President Donald Trump declared states of emergency in Florida, Puerto Rico and the US Virgin Islands.
US Virgin Islands Governor Kenneth Mapp warned that "This is not an opportunity to go outside and try to have fun with a hurricane".
"The dangerousness of this event is like nothing we've ever seen," Ricardo Rossello, Puerto Rico's governor, said.
Florida’s Governor Rick Scott said “we always prepare for the worst and hope for the best and while the exact path of Irma is not absolutely known at this time, we cannot afford to not be prepared.
Trump's homeland security adviser Tom Bossert said the life-saving phase for Hurricane Harvey is over and the government can handle Hurricane Irma relief operations. However, he warned those in the path of Irma to heed evacuation orders.
In regard to Harvey victims, he said the government was designing longer-term assistance, such as Small Business Administration loans, unemployment wages and reconstruction.