Fiji began a massive clean-up on Sunday after the most powerful cyclone in the Pacific nation's history left a trail of destruction, killing six people, flattening scores of homes and crippling infrastructure.

The super-storm lashed the popular tourist destination overnight Saturday, packing wind gusts of 325 kilometres (202 miles) per hour, according to the UN humanitarian agency OCHA.

OCHA said six people were killed during severe tropical cyclone Winston, the first-ever storm system to hit Fiji measuring a maximum category five.

"Homes have been destroyed, many low-lying areas have flooded," Prime Minister Voreqe Bainimarama said earlier Sunday in a statement.

"In the aftermath of this great tragedy, many are without power and full access to water, and are cut off from communication."

Officials said one man died on Koro Island, reportedly from debris sent flying during the cyclone. Others died on Ovalau island and in Tailevu and Ra provinces, National Disaster Management Office director Akapusi Tuifagalele told Fiji One News.

Five others were injured, Tuifagalele added.

OCHA said 150 houses were destroyed and 60 others damaged in the Eastern Division covering the provinces of Kadavu, Lau, Lomaiviti and Rotuma. It gave no figures for the country's three other divisions.

Aid agencies admitted they simply had no idea about the full extent of the destruction, as Fijians shared pictures on social media of roofless houses, flooded streets and metal signposts bent over by the wild winds.

Save the Children Fiji chief Iris Low-McKenzie said it was too early to assess the impact on outlying islands, although unconfirmed reports said thousands of homes had been destroyed and entire villages flattened.

"I'm especially concerned about the remote communities in outlying areas that we haven't been able to contact yet," she said.

"Until communications are re-established and we assess the damage, we won't know the full extent of the situation."

-'Assault on Fiji'-

The capital Suva escaped the full fury of the storm but Low-McKenzie said it was still a terrifying experience.

"I've never experienced anything like this," she said. "The noise was frightening as roofs were blown off homes and trees were ripped out by their roots."

Bainimarama said the storm amounted to an "assault on Fiji", an impoverished nation of about 900,000 heavily reliant on its tourism industry.

"It is being described as one of the most powerful in recorded history... as a nation, we are facing an ordeal of the most grievous kind," he said in a national address late Saturday.

Bainimarama declared a state of natural disaster, to remain in place for one month. A curfew was imposed but would be lifted on Monday morning.

All schools, many of which are being used as evacuation centres, were ordered closed for one week.

Many residents spent the night huddled in the shelters, where they were given food and water.

Falling trees cut power and blocked roads on the main island of Viti Levu, where all flights were cancelled as high winds buffeted Nadi international airport.

Suva resident Danny Southcombe told AFP that while the capital escaped the brunt of the storm, it still came to a standstill.

The military cancelled all leave and mobilised troops for the relief effort.

New Zealand led the international response, sending a P-3 Orion aircraft to help assess damage in remote communities, while Australia offered to do the same.

The acting head of the Red Cross's Pacific office Ahmad Sami said an accurate assessment of the storm's impact would take time.

"We anticipate that humanitarian needs will be very high," he told AFP.

"This is the first time that Fiji has experienced a cyclone of this magnitude in their history, a category five, so we're still trying to find out the figures."

He said priorities were restoring power and repairing damaged homes, as well as maintaining drinking water supplies in more than 700 evacuation centres.

OCHA said hospitals had been badly damaged in Suva and Ba township as Winston "cut a path of destruction across Fiji's islands".

The Fiji Meteorological Office said the cyclone had moved out to sea about 230 kilometres west of Nadi by mid-morning Sunday, although the islands could still expect strong winds, heavy rains and powerful swells.

Airlines Virgin Australia, Air New Zealand and Fiji Airways will resume international flights Monday.

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