Hurricane Irma strengthened to a Category 4 storm as it closed in on the island chain known as the Florida Keys, the US National Hurricane Center said early Sunday.

The storm was churning about 70 miles (115 kilometers) south-southeast of Key West, menacing Florida with maximum sustained winds of 130 miles per hour. 

Its eye was expected to cross the Lower Florida Keys within hours before moving "near or along" the peninsular state's west coast, where it threatened storm surges of up to 15 feet (4.6 meters) -- enough to cover a house