People in Fiji are being warned they may get a direct hit from a powerful cyclone this week.

Cyclone Yasa is now a category four, with a growing likelihood that it will pass directly over Fiji's two main islands on Friday as a category five, the highest possible.

Stephen Meke, a lead forecaster at the Fiji Meteorological Service, said the cyclone already had winds gusting as high as 260km/h.

Cyclone Yasa is forecast to pass close to Fiji's main islands as a category five on Thursday or Friday.

Cyclone Yasa is forecast to pass close to Fiji's main islands as a category five on Thursday or Friday. Photo: Fiji Meteorological Service

He said ocean conditions were perfect for it to strengthen further in the coming days and that 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.

"We should expect a lot of rain, a lot of disaster, a lot of flood, even storm surge to be happening. That is what we are anticipating."

Already, towns and villages are boarding up their houses and stocking up on food and water supplies.

The government also announced that all schools would be closed on Wednesday ahead of the cyclone's expected arrival.

All students between years one and eleven have been asked to stay home, but year 12 and 13 students with exams will still have to sit them.

If Cyclone Yasa does strengthen as forecast, it would be the second category five cyclone in the region this year after Cyclone Harold cause extensive devastation in Vanuatu in April. It is nearly five years since Cyclone Winston - also a category five - wreaked havoc across Fiji.

In a video address, Prime Minister Frank Bainimarama urged people to hunker down and prepare for heavy rain, damagin winds, coastal inundation and flooding.

"Let's remember Cyclone Harold, at the last minute it ramped up in strength and ended up being worse than predicted," he said. "Do not be caught off guard."

"I urge communities to use this time to trim tree branches, clean drains, board up homes, prepare emergency kits and take other steps to keep your homes and communities safe."