Irma now has sustained winds of 290km/h, making it the most powerful the Atlantic has seen in over a decade.
It is projected to bring storm surges, life-threatening winds and torrential rainfall to the Leeward Islands.
Florida, where it is due to arrive as a category four hurricane on Sunday, has declared a state of emergency.
Residents in Texas and Louisiana are still reeling from the effects of Hurricane Harvey, which struck as a category four storm in late August, causing heavy rain and destroying thousands of homes.
