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 Tuesday.

"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.