Vanuatu could soon start running out of food, the president's office warned Tuesday, after Cyclone Pam roared through the Pacific nation leaving at least 24 dead, as a massive clean-up got under way in the capital.

Aid agencies have warned that conditions are among the most challenging they have faced, with mounting concerns about disease, and the nation's President Baldwin Lonsdale has appealed for the world to help.

The full scale of the disaster was unknown with communications to many of the 80 islands in the sprawling archipelago still down and Benjamin Shing, from Lonsdale's office, said survivors would quickly run out of food.

"The first week we are relying on the fact that the food crops and the gardens are still edible and they can be used for the first week, but after the first week we'll need to get some rations on the ground," he told the Australian Broadcasting Corporation.

Flights by military aircraft from Australia, New Zealand and France over Vanuatu's southern island of Tanna, home to some 30,000 people, have confirmed widespread destruction of houses and crops and Shing fears the worst.

"There will be extensive injuries if the people didn't go to higher ground and there might be a lot of fatalities," he said.

Tom Perry from CARE Australia was also concerned.

"The key thing is we still have no contact with other provinces," he said.

"That's of grave concern because there's no real sense from anyone of what the impact has been but we know in the south in particular, it sat under the eye of the storm for hours."

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a situation report that "there are 24 confirmed fatalities" so far. Lonsdale on Monday said there were also some 30 injured.

In the capital Port Vila, access to water and electricity was partially restored after the storm brought down an estimated 80 percent of power lines and damaged up to 90 percent of homes.

Stores also began reopening, but entire neighbourhoods remained without power as aid workers streamed in to help make sense of what many have said was one of the region's worst weather disasters.

As day broke in the capital, stacks of leaves and branches lined the streets while residents began clearing metal roof sheeting from the roads around their homes and using machetes to hack through fallen trees.

Personal belongings, household items, mattresses and clothes were spread out on the ground and hung on washing lines as people dried them out, with the cyclone slowly weakening and posing no further threat to Vanuatu or the South Pacific.

- Concern for children -

Samuel Toara, 25, thought he was going to die when the storm barrelled ashore, sheltering in the pitch black with two other young men as the tempest roared past his home.

"It was very hard. The cyclone sounded like a big plane flying very low," he told AFP.

As heavy rain pounded his house, made of corrugated metal and timber, part of the roof blew off.

"The rain and wind was like white smoke and it flooded up to my knee. But I told the boys don't worry about the water," he said. "As long as we survive."

The United Nations said there were at least 3,300 people sheltering in 37 evacuation centres around the country, including Melissa Song, 22.

She was sharing a small guest room at a Port Vila resort turned into a makeshift evacuation centre with nine other family members, including three young children and a baby.

"We've had no sleep since Thursday," Song told AFP. "We've just been eating tinned tuna, fish and pork."

UNICEF has estimated that 60,000 children have been affected by the cyclone and virtually all schools were closed, with the organisation attempting to restore some normality to their lives.

"We're working to set up temporary learning spaces so kids can start learning and playing again as soon as possible," said the organisation's emergency specialist Mioh Nemoto.

"Food security is likely to be a continual problem and we need to start thinking now about how children will stay well fed."

As aid flights continued landing, workers on the ground said there was no way to distribute supplies across the archipelago's islands, warning it would take days to reach remote villages flattened by the storm.

Save the Children's Vanuatu director Tom Skirrow told AFP the logistical challenges were even worse than for Super Typhoon Haiyan, which struck the Philippines in November 2013, leaving more than 7,350 people dead or missing.