The full extent of the damage is unknown, with limited communications in place after Super Cyclone Pam, a maximum category five storm, slammed directly into the island country late yesterday with gusts up to 320 kilometres (200 miles) an hour.
The UN had unconfirmed reports of 44 people killed in one province, and said late today that there was no clear number of deaths or injuries but that the impact of the cyclone had been "catastrophic".
Aid agencies were scrambling for information and preparing to send teams to Vanuatu - with a UN disaster assessment and coordination team expected to arrive late tomorrow.
"While it is too early to say for certain, early reports are indicating that this weather disaster could potentially be one of the worst in Pacific history," UNICEF New Zealand's executive director Vivien Maidaborn said in a statement.
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Aurelia Balpe, head of the Pacific regional office of the International Federation of Red Cross and Red Crescent Societies (IFRC), said there were unconfirmed reports of casualties in the capital Port Vila.
"We are starting to get a picture from Port Vila, but there's nothing from the south," she told AFP from Suva.
"We are very worried just because there are less permanent structures in that part of the country. There's probably less infrastructure to be evacuated to, it's a less developed part of the country."
"The scene here this morning is complete devastation - houses are destroyed, trees are down, roads are blocked and people are wandering the streets looking for help," said Save the Children's Tom Skirrow.
UNICEF spokeswoman Alice Clements described the cyclone as "15-30 minutes of absolute terror" for "everybody in this country" as it passed over.