Vanuatu has hit out at aid groups swarming the cyclone-ravaged Pacific nation over a lack of coordination, which it said cost precious time getting help to those in need, while warning food will run out in a week.

Relief agencies have been battling logistical challenges in the sprawling archipelago with a lack of landing strips and deep water ports hampering their efforts to reach distant islands and get a better grip on the full scale of the disaster.

They continue to paint a bleak picture, detailing large-scale property and crop destruction, and an urgent need for clean water, medical supplies, tents, bedding and hygiene kits.

Aid finally reached the badly hit island of Tanna on Wednesday, five days after Severe Cyclone Pam roared ashore on Friday night, but many of the 80 islands that make up Vanuatu remain without help.

National disaster committee deputy chair Benjamin Shing said while the country appreciated the aid, the initial response could have been handled better with many groups and NGOs working on their own rather than in cooperation with the government.

"I do apologise but I have to state the facts. We have seen this time and time again," he said at a briefing late Wednesday in the capital Port Vila.

"In nearly every country in the world where they go in they have their own operational systems, they have their own networks and they refuse to conform to government directives.

"We had to spend the first three days trying to get some form of coordination in place. That was much precious time that could have been spent doing the assessments instead."

Oxfam country director in Port Vila Colin Collett van Rooyen denied any disorganisation.

"Our position is that we will continue to work with the government, as we have been, and as we always do, to address the best interests of those in need," he told AFP.

In a situation update, the UN Office for the Coordination of Humanitarian Affairs (OCHA) noted: "Coordination with the ministry of health regarding flights to provinces is critical".

- Death toll confusion -

There is also confusion over the death toll. Early reports from OCHA on Friday evening announced an unconfirmed 44 dead, which then dropped to 24 confirmed, only to be revised down again to 11.

The Vanuatu government said in fact only seven people were confirmed dead and that the four others were patients already in hospitals whose deaths were not directly related to the storm.

"The number of confirmed fatalities that we have for Tanna, there are five, for Port Vila there are six, so if you do the math, it's 11," said Shing.

"But technically, there are only seven that are due to the cyclone. Four deaths in the hospital were patients... they were already sick. Not related to the cyclone -- unrelated deaths."

Reconnaissance flights by military aircraft from Australia and France have "found severe and widespread damage across the larger islands of Tanna, Erromango and Efate," the UN said, but less destruction on the nation's smaller southern islands.

Van Rooyen said work was under way Thursday to identify priority areas across the archipelago and get aid to them as soon as possible.

Efforts to ferry relief supplies to Tanna and other islands continued, with New Zealand sending a navy ship to Vanuatu with heavy engineering equipment to help repair roads and key infrastructure.

France also dispatched a frigate from New Caledonia with helicopters on board.

Agriculture Minister David Tosul warned the struggling subsistence-based country needed rice, biscuits, seeds, tinned protein and cash to help ship them in.

He said bananas and other fruit trees had been destroyed, leafy vegetables devastated and staple root crops irreparably damaged, with limited supplies of imported food.

"In short, our agricultural experts estimate that Vanuatu's people will run out of food in less than one week," he said.