Aid began arriving in some of cyclone-hit Vanuatu's worst affected islands Wednesday but others remain isolated, with flights over the Pacific nation showing desperate villagers spelling out the letter "H" for help.

Relief agencies are 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.

Vanuatu Prime Minister Joe Natuman, who travelled to hard-hit Tanna island on Wednesday on a New Zealand C-130 Hercules, said "it'll be at least a week or two" before the full impact of Severe Tropical Cyclone Pam is known.

The UN Office for the Coordination of Humanitarian Affairs revised its death toll down to 11 from 24 but said it was expected to rise, while the Vanuatu government puts the toll at seven.

Aid groups continued to paint a bleak picture, warning of large-scale property destruction and shortages of food and clean water.

"The yams are rotting in the mud. There's no more bananas, fruit, anything. Pam took everything," said Philemon Mansale, the head of a large family from Mele village outside the capital Port Vila on the main island of Efate.

The southern islands of Tanna and Erromango bore the full brunt of the cyclone when it barreled in late Friday, and Oxfam, the UN and CARE Australia said assessments showed widespread devastation with entire villages destroyed.

"In Tanna at Lenakel, the provincial capital, 70 percent of houses are damaged," CARE's Tom Perry told AFP from Port Vila.

The whole township of Waesisi on Tanna's northeast coast was "inundated with water... and 100 percent damaged".

An AFP photographer on Tanna said the land was stripped bare of coconut trees, wells were swamped by landslides and doctors at the island's hospital worked on patients while wading through mud-covered floors.

"I've never seen nothing like it, just the noise and the destruction," said Australian tourist Andrew Brooks, who felt the cyclone's fury as he sheltered in a makeshift evacuation centre in a school on Tanna.

- 'Wind was screaming' -

"You could see the wind -- it was white with rain and debris. It was horizontal. The wind was screaming, trees were crashing and sheets of tin and debris were flying. People were cowering. It pretty much lasted all day like that."

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

Aid teams reached Tanna, home to 30,000 people, on Tuesday and at least seven tonnes of supplies from organisations such as the Red Cross and the New Zealand military arrived Wednesday, including medical supplies, sanitation kits, tarpaulins, and chainsaws.

A ferry full of aid was expected Thursday as the humanitarian response moves up a gear while coconut oil producer COPSL offered its six, 60-tonne Vanuatu-based vessels to help distribute supplies to other islands.

Benjamin Shing, from Vanuatu President Baldwin Lonsdale's office, said aid was due to reach smaller islands in the central province of Shefa on Thursday, while communications were being restored around the nation.

Many of the archipelago's 80 islands remain cut off and Oxfam country director in Port Vila Colin Collett van Rooyen said flights over some of them saw people signalling for help.

"The aerial assessments of Ambryn island reported large white 'Hs' marked out on the ground by people signalling for help, and on Tongoa island people holding up mirrors also signalling for help," he said.

While the death toll was revised down, he said real concerns remained about disease with water contaminated, sanitation equipment destroyed and an increasing lack of food.

A BBC team reached the small island of Moso in the northwest and reported that people had resorted to drinking harmful saltwater, with outside help yet to arrive. Drinking saltwater can lead to dehydration and death.

Despite the challenging circumstances, many Vanuatuans remain optimistic, living up to their resilient reputation.

"We feel sad. But we (neighbours) always sit together, and that makes me happy," said Lida Chilia, a villager from Mele on how she and her friends were coping.