Cyclone Yasa is now a category 5 storm, the highest possible level, and there is a growing likelihood it will pass directly over Fiji's two main islands late Thursday or early Friday.
The storm is packing average winds of around 200 km/h with gusts up to 280 km/h.
Stephen Meke, a lead forecaster at the Fiji Meteorological Service, said ocean conditions were perfect for it to strengthen further and people should heed warnings.
"All the ingredients for further intensification is there," Meke said. "This is massive for Fiji, it will be very destructive for Fijians.