The storm was downgraded as it moved north towards Atlanta, with maximum sustained winds of 35mph later recorded, the National Hurricane Center (NHC) said in a statement.
The NHC statement said that while heavy rain was expected to continue across south-eastern states, all storm surge and tropical storm warnings had been discontinued.
Media reports link at least four deaths to the storm in Florida. Last week it killed at least 37 people in Caribbean islands.
Meanwhile, White House Homeland Security Adviser Tom Bossert has announced that it will be some time before residents in the Florida Keys are able to return to their homes.
“I would expect that the Keys are not fit for re-entry for regular citizenry for weeks,” he said.
Speaking as he went on an aerial tour of the Keys, Florida Governor Rick Scott said: “Power lines are down throughout the state. We’ve got roads that are impassable, so everybody’s got to be patient as we work through this.”
The Keys are cut off from the mainland, as the 42 bridges that link them are being assessed for damage. According to reports, 10,000 people decided to ride out the storm.
Irma has strengthened to a category 4 hurricane as it approaches Florida, with maximum sustained winds of 130mph.
Wind gusts close to hurricane force are already battering islands in south Florida, with the mainland due to be hit in the coming hours.
Water levels are already rising on Florida coast, where a huge storm surge is expected.
At least 25 people died when hurricane Irma earlier hit several Caribbean islands.
In Florida, 6.3 million people – about 30% of the state’s population – had been told to evacuate. However, on September 9, Florida Governor Rick Scott said it was now too late to leave for anyone remaining.
The National Hurricane Center (NHC) warns that a “life-threatening storm surge” is expected in the Florida Keys – a chain of small islands in Florida’s south – and also the west coast of Florida.