Relief workers rushed to deliver desperately needed food and water Wednesday to survivors living on Vanuatu's outer islands, after a monstrous cyclone wiped out entire villages and flattened vast swathes of the South Pacific nation's landscape.

Aid workers and government officials were planning to send a boat packed with supplies to hard-hit Tanna Island, where aerial assessments showed more than 80 percent of homes or buildings had been partially or completely destroyed by Cyclone Pam.

"There's a landscape of skeleton trees and patchworks of square outlines where houses used to be," said Angus Hohenboken from aid group Oxfam. "It's really quite a saddening sight."

Lack of food was a growing worry for those who survived the storm, which packed winds of 270 kilometers (168 miles) per hour when it struck Saturday.

"Everyone in Tanna and other islands in the south, they really live subsistence lives, so they grow what they need for a short period. ... And the reality is that much of that would have been washed away by this storm," said Tom Perry, spokesman for CARE Australia. "That's a grave concern because we desperately need to get food to people soon."

The U.N. Office for the Coordination of Humanitarian Affairs reported that 11 people were confirmed dead, including five on Tanna. Officials with Vanuatu's National Disaster Management Office said they had no accurate figures on how many were dead, and aid agencies reported varying numbers.

Many people took shelter in larger buildings such as schools, which likely spared their lives.

Aid workers carrying medical and sanitation supplies, water, food and shelter equipment finally managed to reach Tanna and neighboring Erromango Island, after being stymied in their efforts for days by poor weather and a breakdown in communications. The two islands were directly in the path of the storm.

An aerial assessment showed extensive damage on Erromango, with communities ranging from 70 percent to 100 percent destroyed on the archipelago's fourth-largest island. On other islands, plane crews saw people had made big, white "H'' marks on the ground, and people on Tongoa island flashed mirrors to attract attention, said Colin Collett van Rooyen, Vanuatu director for aid group Oxfam.

On Tanna, the cyclone's fierce winds uprooted water tanks and blew them kilometers (miles) away, said Hohenboken from Oxfam, who traveled to the island. Crops were demolished and electricity was out, as the solar panels that power many homes were destroyed.

The island's hospital was operating with a diesel generator, but there was only enough fuel to last for two weeks, and some of the hospital's water supply was unusable due to contamination, he said.

On Wednesday, some residents were already beginning the process of rebuilding their houses and some had started re-planting crops, Hohenboken said.

"I think Vanuatuans are very resilient and kind of motivated people, but it's been a shock to this community," he said. "It was a storm on a scale that people really had no frame of reference for."

Meanwhile, fears of a measles outbreak prompted aid workers to launch an emergency vaccination drive for children across Vanuatu, which has low rates of immunization and already suffered one outbreak of the disease earlier this month. Teams were traveling to evacuation centers and other storm-ravaged areas around Port Vila to vaccinate children, provide Vitamin A and hand out bed nets to help stave off mosquito-borne malaria, according to UNICEF.

Baldwin Lonsdale, Vanuatu's president, returned to his country on Tuesday night from Japan, where had been attending a U.N. disaster conference when the cyclone struck.

"I trust the people of Vanuatu. I trust my government. I trust the people that they will stand united together as a nation and to rebuild the nation," he said.

Poor weather and communications issues have hampered relief workers efforts to reach the outer islands for days. Most of the islands have no airports and those that do have only small landing strips that are tricky for large supply planes to navigate. On the main island of Efate, bridges were down outside Port Vila, impeding vehicle traffic.

"There are over 80 islands that make up Vanuatu and on a good, sunny day outside of cyclone season it's difficult to get to many of them," said Collett van Rooyen of Oxfam.

Vanuatu has a population of 267,000 people. About 47,000 people live in the capital.

The U.N. Office for the Coordination of Humanitarian Affairs said 3,300 people were sheltering in dozens of evacuation centers on the main island of Efate and in the provinces of Torba and Penama.

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Associated Press writer Kristen Gelineau in Sydney contributed to this report.