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 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.

It's still impossible to pinpoint where Irma will make landfall, and the states of Georgia, South Carolina and North Carolina need to join Florida in monitoring the path of the massive hurricane, he said.

"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.

"We are already in good communication with all of those states, and working with them to be prepared," Long told Fox News' "Fox & Friends" program, noting that he also has been in communication with President Donald Trump and Vice President Mike Pence came

"Just like we had in Harvey, the president stands ready to support disaster declaration requests and communication lines are open," said Long. "He has given me the authority I need to move forward."

Irma promises to be a completely different storm than Hurricane Harvey, which slammed into Texas late last month, said Long.

"Harvey was a rapidly intensifying storm that did bring Category 4 winds for a small duration for the Texas coast, but then lost steering current for the next five to six days which means copious amounts of rainfall," said Long. "Irma has good steering current. It's moving at 17, 18 miles per hour, so right now, the major hazard that people are going to see that we evacuate them for is coastal flood inundation, or basically wind-driven water coming up on shore, knocking houses down."

In Texas, FEMA has established recovery commands, and has registered more than 600,000 people in support of individual assistance, said Long.

"We can't make them whole, but we are providing services and funding to individuals that qualify through our systems to help them out," said Long. "The other thing that we are doing is racing to pick up debris and we are also trying to get people into livable situations."

Many evacuations already are occurring in Florida, and orders need to be heeded, Long said, but FEMA does not make evacuation decisions.

"The goal is get out of a storm surge area and into a facility that can withstand the winds," said Long. "That's the goal."

"If there was any issues, we would be putting it out to the citizens as quickly as we can," said Long. "Right now I think we are good."

On CNN, Long said on the network's "New Day" program that there are nearly 3,000 federal workers in place in the Virgin Islands and Puerto Rico, as well as in the states in Irma's path.

"Right now, we just need people to heed the evacuation warnings being issued in the state of Florida," said Long. "I expect more of those warning orders to be issued by local and state officials in Georgia, South Carolina, North Carolina, over the next 24, 48 hours, so the entire southeastern United States better wake up and pay attention."