Hurricane Irma slowed to a Category Two storm after making its second landfall in Florida on Sunday, US forecasters said, warning it still had "dangerous" winds causing "life-threatening" storm surges.

At 5:00 pm (2100 GMT), the hurricane had top winds of 110 miles (177 kilometers) per hour -- at the top end of the Category Two storm range -- with the eye of the storm located just five miles (eight kilometers) north of Naples, the National Hurricane Center said.

"Dangerous storm surges expected immediately after the eye passage along the Florida West Coast," the NHC said. "This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions."

It cautioned that although it was set to weaken further, Irma was expected to remain a hurricane at least through early Monday.