Vanuatu is in "immediate" need after Cyclone Pam tore through the country at the weekend, its president says.
President Baldwin Lonsdale said the storm had "wiped out" all development of recent years and his country would have to rebuild "everything".
Aid has begun arriving in the storm-hit nation - one of the world's poorest - but contact has still not been made with some of its more remote islands.
Aid agencies say it could be one of the worst disasters ever to hit the region.
The official death toll stands at eight, but it is expected to rise.
The BBC's Jon Donnison, in the capital Port Vila, says just about every house there has received some damage and the situation for many people is bleak.
One village chief told our correspondent there was a desperate need for fresh water supplies.
At the scene: Phil Mercer, BBC News, Port Vila
The sense of devastation is absolutely immense and when you land it doesn't take long for that sense of devastation to increase.
Many family homes have been stripped of their roofs or flattened by very powerful winds and torrential rain.
The air here is very thick with smoke because the cleanup has already begun - the debris is being chopped down, collected and burned. There is a sense here that people will rebuild but it only takes a brief moment in the capital to realise that this rebuilding effort will take many months if not years.
This is a vast archipelago, the population is spread over more than 60 islands and communications are down. It's very difficult for the authorities to have a true picture of the devastation but it's clear the number of dead will increase when communications are made with those outlying areas.
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).
President Lonsdale, who was attending a disaster preparedness conference in Japan when the storm hit, has appealed for international help, telling AFP news agency: "The humanitarian need is immediate, we need it right now."
"After all the development we have done for the last couple of years and this big cyclone came and just destroyed all the infrastructure the government has built. Completely destroyed."
Earlier, Mr Lonsdale had described the storm as "a monster", and said he had not been able to confirm that his own family was safe.
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.
Unicef worker Alice Clements in Port Vila said all the power lines were down, and many staff of the capital's only hospital were unable to get in to work.
She told the BBC a major bridge connecting Port Vila to the eastern part of the island looked like it had "a huge bite taken out of it".
"This isn't just some rickety bridge, it's made of concrete and steel, and now there's just steel girders sticking out. It really shows the sheer force of the storm."
How poor is Vanuatu?
- The UN considers it one of world's least developed countries. It has a GDP of $828m (£560.7m) compared to neighbouring Australia's $1.56tn, according to the World Bank.
- About two-thirds of people make a living from agriculture. Fishing, tourism and offshore financial services are the other main industries.
- Australia estimates that about 70% of the population of 250,000 live on remote islands or in rural areas, with few services and limited access to clean drinking water, transport or electricity.
- Australia is Vanuatu's main donor, giving A$60.7m (£31.45m: $46.5) in 2013/4, about 60% of total aid.
There is major concern about islands to the south, including Tanna, which was in the direct path of the storm. The Australian Red Cross tweeted that it had heard of "utter devastation" there.
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.
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.
In a statement on Sunday, Oxfam Australia said this was "likely to be one of the worst disasters ever seen in the Pacific".
World Vision said it had not been able to contact more than 30 of its staff.
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.
It is now moving down the east coast of New Zealand. The storm has weakened significantly but about 100 people have been evacuated from coastal areas in Gisbourne, on the east of the North Island. An alert is also in place in the Chatham Islands, about 680km south-east of Wellington.