"It felt like the world was going to end," Alice Clements, a spokeswoman for the UN Children's Fund (UNICEF), said from Vanuatu.
"It's like a bomb has gone off in the center of the town. There is no power. There is no water."
Tom Skirrow, country director for the Save the Children aid group, told Reuters that Vanuatu's National Disaster Management Office had confirmed eight dead and 20 injured. He said he expected those figures to rise substantially.
Aid workers in Papua New Guinea said at least one person had been killed by the storm there.
Satellite photographs showed the storm covering virtually all of Vanuatu, a sprawling country of 83 islands and 260,000 people 2,000 km (1,250 miles) northeast of the Australian city of Brisbane.
The president of Vanuatu, Baldwin Lonsdale, told a disaster risk conference in Japan he had no confirmed report of the impact of the storm but he appealed to the world to "give a lending hand".
Australian Foreign Minister Julie Bishop said Canberra would be willing to offer Vanuatu whatever help it could.
Formerly known as the New Hebrides, Vanuatu was jointly ruled by France and Britain until independence in 1980. It is among the world's poorest countries and highly prone to natural disasters, such as earthquakes, tsunamis and storms.
Witnesses described sea surges of up to eight meters (26 feet) and flooding throughout the capital, Port Vila, after the category 5 cyclone hit late on Friday.
Aid officials said the storm could be unprecedented in the island's history and one of the worst natural disasters the Pacific region has ever experienced.
They said the storm was comparable in strength to Typhoon Haiyan, which hit the Philippines in 2013 and killed more than 6,000 people.