Aid workers were particularly worried about the southern island of Tanna. An official with the Australian Red Cross said an aircraft had managed to land there and confirmed "widespread destruction".

"Virtually every building that is not concrete has been flattened," said the official, adding two deaths had been confirmed on the island which has a population of about 29,000 and is about 200km south of the capital, Port Vila. Witnesses in Port Vila described sea surges of up to eight metres and widespread flooding as the category 5 cyclone hit.

Residents said the storm sounded like a freight train. Port Vila was strewn with debris and looked as if a bomb had gone off.

President Baldwin Lonsdale, who happened to be at a disaster risk conference in Japan, likened the storm to a monster.

"Most of the houses in Vila ... have been damaged and destroyed. People are finding shelter where they can live for the night," he said.

He said the impact would be "the very, very, very worst" in isolated outer islands but held out hope the number of casualties would be "minor".

He said offers of aid had been "very magnanimous".

"We are not begging, but we are asking for assistance."

Vanuatu's climate change minister, James Bule, said people were used to storms, though not usually such strong ones, and he also hoped loss of life might be limited.

Formerly known as the New Hebrides, Vanuatu is a sprawling cluster of 83 islands and 260,000 people, 2,000km northeast of the Australian city of Brisbane. It is among the world's poorest countries and highly prone to disasters such as earthquakes, tsunamis and storms.

Aid officials said the storm was comparable in strength to Typhoon Haiyan, which hit the Philippines in 2013 and killed more than 6,000 people, and looked set to be one of the worst natural disasters the Pacific region had ever experienced.

Kris Paraskevas, a consultant in Port Vila, said the situation was catastrophic.

"The villages are no good. Many houses were just poles and tin or thatch. There's nothing left, people are just sitting in rubble," Paraskevas said.

Aid flights, including a New Zealand military Hercules aircraft carrying eight tonnes of supplies and an initial team, landed yesterday as Port Vila's airport partially reopened.

Australia sent two military aircraft including one with medical experts, search and rescue teams and emergency supplies, while a UN team was also preparing to go in with members drawn from as far away as Europe.

Oxfam's country manager Colin Collett van Rooyen said the outlying islands were particularly vulnerable.

"We are talking about islands that are remote … with none of what we would call modern infrastructure," he said.

Late yesterday Pam weakened, moving to the southeast.