The number of people in the US state of Florida under orders to evacuate in the face of approaching Hurricane Irma has climbed to 6.3 million, authorities said Saturday.

The increase means an additional 700,000 million people have been told to get moving since Friday, when state officials said 5.6 million people were under evacuation orders.

The state's division of emergency management said it "estimates that 6.3 million Floridians have been ordered to evacuate."

The state has a population of 20.6 million, and with the massive Category Four storm forecast to move south to north Florida Governor Rick Scott has said everyone should be ready to evacuate.