Cyclone Pam has "wiped out" development in Vanuatu, its president said, amid ongoing concern over residents of the Pacific nation's outlying islands.
Aid has begun arriving in the storm-hit nation - one of the world's poorest - where trees and power lines have been felled and many buildings destroyed.
Aid agencies say it could be one of the worst disasters ever to hit the region.
The situation on some islands closer to the eye of the storm is not yet known as communication networks are down.
The BBC's Jon Donnison, who has just arrived in the capital, Port Vila, says just about every house has received some damage.
Aerial images of the capital show houses completely flattened, and aid workers have said there is an urgent need for fresh water supplies.
At least eight people are confirmed to have died but the death toll is expected to rise as rescuers reach more the remote islands.
Speaking in Japan where he had been attending an international conference on disaster preparedness, President Baldwin Lonsdale called the storm "devastating".
"I term it as a monster, a monster - it's a setback for the government and for the people of Vanuatu," he said. "After all the development that has taken place, all this development has been wiped out."
He also said he had not been able to confirm that his own family was safe. "As the leader of the nation, my whole heart is for the people, the nation."
The president said climate change had contributed to the disaster, saying his country had seen changing weather patterns, rising seas and heavier-than-average rain.
Pacific nations have repeatedly said they will be among the first victims of climate change, though scientists say this is not necessarily the cause of individual powerful storms.
The category five storm, with winds of up to 300km/h (185mph), struck populated areas when it reached Vanuatu early on Saturday local time (+11 GMT).
It is now moving down the east coast of New Zealand, but has weakened significantly.
In Port Vila, concrete buildings withstood the storm better but reports suggest as many as three-quarters of the capital's houses have been damaged or destroyed.
Government buildings, bridges and the main hospital have also been hit.
"There are houses that have been totally gutted, literally with a couple of timber frames still standing," Chloe Morrison, from the international children's charity World Vision, told the BBC.
There is major concern about islands to the south, including Tanna, which was in the direct path of the storm.
Many people in the region live in basic housing with limited communications which have now been severed.
Paolo Malatu, co-ordinator for the National Disaster Management Office, told the Associated Press news agency that planes and helicopters had been sent to fly over the islands and assess the situation.
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British doctor Carina Smith said healthcare facilities had been damaged
Map showing islands in the South Pacific at threat from Cyclone Pam - 13 March 2015
Military planes carrying supplies have started arriving from Australia and New Zealand, and other countries have pledged to help. Commercial flights were due to resume on Monday.
Squadron Leader Leigh Foster from the New Zealand Air Force told the BBC they were working to deliver what was needed.
"We've delivered a forklift from the Australians, we've got medical personnel going up on the flight right now to repatriate New Zealand citizens and we've also sent up personnel and equipment from other government agencies."
In a statement on Sunday, Oxfam Australia said this was "likely to be one of the worst disasters ever seen in the Pacific".
"The scale of humanitarian need will be enormous," said Oxfam's Colin Collet van Rooyen in Port Vila.
Pam had already caused major damage on other Pacific islands, including Kiribati and the Solomon Islands. Tuvalu declared a state of emergency after the cyclone caused flash floods there.
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