Hurricane Irma was downgraded to a Category Three storm on Sunday afternoon, forecasters said, but they warned that it remained "powerful."

Maximum sustained winds were at 120 miles per hour (195 kilometers per hour), with higher gusts, the National Hurricane Center said as the storm slowly approached mainland Florida.

It had whipped the Florida Keys island chain early Sunday as a Category Four storm, packing maximum sustained winds of 130 miles per hour.

"While weakening is forecast, Irma is expected to remain a powerful hurricane while it moves near or along the west cost of Florida," the center said