MIAMI (AFP) - Irma destroyed a quarter of the houses in the Florida Keys, where it first made landfall in the United States as a Category Four hurricane, the US emergency response chief said on Tuesday (Sept 12).

"Some of the initial estimates are - and this is why we asked people to evacuate, largely from storm surge - 25 percent of the houses in the Keys initially have been destroyed and 60 percent have been damaged," Fema director Brock Long said.

"Basically every house in the Keys has been impacted some way or another," Long told a news conference.

Keys residents were just beginning to return but most of the low-lying archipelago south of Miami remains closed to traffic as authorities assessed conditions.

Aerial views and television images show extensive damage awaits them in what in normal times is a palmy haven for tourists, boaters and scuba divers.

Irma made landfall Sunday on Cudjoe Key in the lower part of the Keys with winds of 209kmh.