Super cyclone Yasa flattened entire villages as it tore through Fiji, aid agencies said Friday, with a baby among two confirmed deaths and rescue workers racing to the worst-hit communities.

Yasa made landfall at the Pacific Island nation late Thursday as a top-of-the-scale Category Five cyclone, lashing Fiji's second-largest island, Vanua Levu.

The storm triggered floods, landslides and blackouts before moving out to sea early Friday, where it rapidly weakened to a Category Three system.

Zalim Hussein of Savusavu, a small town of a few thousand people on Vanua Levu, said he feared for his life sheltering at home in the dark as screeching winds ripped apart houses around him.

"I could hear roofs of neighbouring houses flying, trees falling and branches breaking outside and big waves crushing on the shore," he told AFP.

"We were all scared for our lives and I thought at one point we'd lose our home. In my 65 years, I've never seen anything like this.

Fiji Prime Minister Frank Bainimarama said two deaths had been confirmed, a 45-year-old man and a three-month-old baby. 

"We sadly expect fatalities to rise," he said.

Most dwellings in rural Fiji are made from timber and corrugated iron sheeting, and are not made to withstand winds like those unleashed by Yasa, which had been forecast to bring gusts of up to 345 kilometres an hour (210 miles an hour).

"There's quite a few villages that are reporting that all homes have been destroyed," Save the Children's Fiji chief Shairana Ali told AFP.

"Most of these people rely on farming for their livelihood and their crops have been destroyed as well."

'Climate emergency'

The Red Cross said it was scrambling response teams amid "extensive destruction" in Vanua Levu's Bua region and coastal communities inundated by storm surges.

Aid agencies had pre-positioned supplies across the country in anticipation of major disasters during cyclone season, which runs until May.

Bainimarama said there were about 24,000 people sheltering in almost 500 evacuation centres across the country.

Authorities had issued dire warnings about the danger posed by the cyclone for most of the week, urging people to find solid structures or flee to higher ground if they live on the coast.

A state of natural disaster was declared on Thursday, giving emergency services sweeping powers to impose curfews and movement restrictions for the next 30 days.

Yasa is the third Category Five storm to hit Fiji since 2016, when Cyclone Winston killed 44 people and destroyed tens of thousands of homes.

The most recent was Cyclone Harold, which claimed 31 lives as it tracked through the Solomon Islands, Vanuatu, Fiji and Tonga in April this year.

"It's devastating to see another big cyclone affect Fiji so soon after Cyclone Harold and so close to Christmas," the Red Cross's Suva-based Pacific chief Kathryn Clarkson said. 

"With communities that are already facing challenges because of Covid-19 this will only add to the hardships."

The human cost of Yasa could have been worse had it not landed in the sparsely populated Bau province, causing no major damage to large towns, except for flooding in Rakiraki on the main island of Viti Levu.

However, Save the Children's Ali said the full picture of the storm's impact was yet to emerge from isolated rural communities and remote islands.

Bainimarama, a long-time campaigner for climate action, blamed global warming for creating the recent monster storms, which were once rare but have become relatively common.

"This is not normal. This is a climate emergency," he tweeted.