Cyclone Pam had destroyed most buildings in Vanuatu’s capital Port Vila, including schools and clinics, said President Baldwin Lonsdale.
A state of emergency has been declared in the tiny Pacific state of 267,000 people, spread over 65 islands.
At least eight people are reported to have been killed.
However, it is feared the toll will rise sharply as rescuers reach outlying islands.
Thousands of people spent a second night in shelters.
Cyclone Pam, with winds of up to 170mph, veered off its expected course and struck populated areas when it reached Vanuatu early on March 14.
The authorities in Vanuatu are struggling to gauge the extent of the damage across the country because communications are down and fallen power lines and trees have blocked roads.
Meanwhile, the first deliveries of aid have arrived on air force planes from New Zealand and Australia.
The UK, France, UN and European Union have also promised help.
A UN disaster assessment team is due to arrive in Vanuatu in the coming hours.
Chloe Morrison, an emergency communications officer with World Vision in Port Vila, said residents had woken to much calmer weather on March 15.
She said it would have been a “very, very tough time” for anyone not in a secure shelter during the cyclone.
Speaking from Japan, President Baldwin Lonsdale made an impassioned plea for international help earlier.
“I am speaking to you today with a heart that is so heavy,” he said at the UN Conference on Disaster Risk Reduction.
“I stand to appeal on behalf of the government and the people to give a helping hand in this disaster.”
The extent of the devastation is unlikely to be known for several days, said Tom Skirrow, Save the Children’s Vanuatu country director.
Unconfirmed reports on March 14 said 44 people had died in Penama province in the north-east of Vanuatu, according to the UN’s Office for the Co-ordination for Humanitarian Assistance (UNOCHA).
Cyclone Pam had already caused major damage on other Pacific islands, including Kiribati and the Solomon Islands.
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