Two New Zealand defence force flights taking aid supplies to Vanuatu have had to turn back because of technical problems.
Both a Hercules and a Boeing-757 developed problems with their instruments on the way to Port Vila.
Head of Air Force operations Air Commodore Kevin McEvoy said safety protocols meant both planes had to immediately return to their base at Whenuapai near Auckland.
"Naturally we're very disappointed that we can't continue rendering assistance today," he said. "However, the aid on board will be delivered as soon as possible."
The Air Force is trying to reschedule the flights for another day.
There have already been three Air Force flights to Vanuatu, which have delivered about 32,000 kilograms of aid supplies and 42 aid workers.
While relief supplies have been arriving by military aircraft, aid workers on the ground have warned of difficulties in distributing supplies across the country's many islands and say it will take days to reach remote villages flattened by the huge storm.
Vanuatu's Disaster Management Office spokesperson Mishael Lulu Garae said the government had enough food supplies for evacuation centres in Port Vila to last the rest of the week, but international food assistance will be needed.
"At the moment it's really the Vanuatu government who's looking after the rations at the evacuation centres here in Port Vila. However, in the next like week or so after that they would be needing external support for food needs," said spokesman Mr Garae.
The government estimated about 80 percent of homes in the city had been either destroyed or sustained significant damage, he said, though it was not yet clear how many people in the town of about 40,000 were homeless.
Some without basic supplies
Save the Children says several thousand people are living in 28 evacuation centres it had set up in churches and other buildings that survived the cyclone.
Spokesperson in Port Vila Sally Page said many people had been left without the most basic supplies.
"Yesterday our teams here distributed 1,800 kits of food and water and cleaning supplies. And we hope to be able to do the same today."
But Ms Page said it could be days still before it and other aid agencies can reach remote areas of Vanuatu, as no one had been able to contact outlying islands.
"We're trying daily to establish comms, and as I said there are technicians coming in to try and step comms up. So that may be the way that we get out to the outer islands, to get the aid to the children or families that need it the most," she said.
Alice Clements, a coordinator for Unicef, said any donations at this stage would save lives.
However aid workers feared lives were at serious risk from a measles outbreak in the aftermath of the cyclone
Thousands of people are living in close confines in evacuation centres set up in the capital Port Vila
The country was already experiencing a measles outbreak and six vaccination teams are now in Port Vila trying to immunise as many people as possible to make sure the disease doesn't spread in the post-disaster conditions.
Two more NZDF aircraft were scheduled to leave Auckland today with emergency supplies.
The European Commission will also contribure more than US$1 million to the emergency relief effort.
The Commission would also send two experts to assess the humanitarian needs and liaise with partner organisations operating on the ground.
The French territories in the Pacific are sending aid to cyclone-hit Vanuatu, as part of the FRANZ agreement between France, Australia and New Zealand.
A French military plane travelled to Port Vila on Monday carrying tarpaulins to shelter a hundred families, as well as chainsaws, a satellite unit, a logistics unit and seven emergency staff.
Food crops severely impacted
Unicef said many communities in Vanuatu had just a week before they completely run out of food.
The agency said fresh water was the most urgent need in the wake of cyclone Pam, but food was close behind.
Unicef spokesperson in Port Vila Alice Clements said the reports they were getting suggest some island communities, which rely on subsistence farming, had lost their entire crop.
"They were saying they would have fruits from the trees that might last them a few days ... And there might be access to root vegetables in the ground that may or may not have been destroyed by the water.
"But after that, that's about a week's worth of food. After that, they have no food. And we growing the crops here will take at least three months."
A Vanuatu agriculture expert said the damage to staple food crops from Cyclone Pam throughout much of the country was very serious.
Vincent Lebot from the French research centre CIRAD in Port Vila said Pam's path covered a broad arc across many of Vanuatu's most populated islands, going through Torres and Banks, and then south past Maewo and Pentecost, before whipping through the Shepard islands and passing Erromango and Tanna.
"So basically an important part of the population of Vanuatu is directly affected, and most of these islands of course had plantations and food gardens and crops and they are of course very severely impacted."
Dr Lebot said people on these islands live on their food gardens, crops like fruit and nut trees, or root crops.
He said two days after the cyclone left the country, it was becoming evident that there was little to eat.
"The bananas are on the ground and the root crops which are the staples are all gone. I mean, the cassava will go first of all and the yams were not ready yet so there is nothing to harvest. Only the taros are left," Dr Lebot said.
Appeal for more aid
Aid agencies in Vanuatu are appealing for food and water, following the devastating cyclone that has left much of the country uninhabitable.
The International Federation of the Red Cross and the Red Crescent Society has launched an appeal to raise funds to help the victims of the damage cyclone in Vanuatu.
The Geneva-based organisation wants to raise just over a million US dollars.
Oxfam says there is an incredible sense of anticipation and urgency.
Contact has been made with the heavily populated Tanna, a main island south of the capital, where aid agents on the ground are reporting severe damage.
But a Save the Children spokesperson in Port Vila, Sally Page, said no one had been able to contact other outlying islands.
Unicef's Alice Clements said any donations at this stage would save lives.
"We urgently need water purification tablets to make sure that the water is safe to drink. We urgently need food for communities whose crops have been absolutely destroyed and will take up to three months to grow.
"We absolutely need shelter, we absolutely need support to make sure medical care is availiable to those who need it."