The hurricane has sustained winds of up to 215 km/h and is likely to strengthen in the next 48 hours, the US National Hurricane Center (NHC) said.
Irma is projected to hit the Leeward Islands, causing storm surges, life-threatening winds and torrential rain.
The US state of Florida has declared a state of emergency.
It comes as residents in Texas and Louisiana are reeling from the effects of Hurricane Harvey, which struck as a category four storm, causing heavy rain and destroying thousands of homes.
