A distress signal was detected in two low-lying islands in the Tonga archipelago, the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) says.
Hunga-Tonga-Hunga-Ha'apai volcano erupted late on Saturday afternoon, causing a tsunami and covering much of Tonga in ash.
OCHA on Tuesday reported "significant infrastructural damage around Tongatapu, the main island", while "extensive damage to property has also been reported on the western beaches of Tongatapu with several resorts and houses destroyed and/or badly damaged".
Much of the communication network in Tonga went down following the natural disaster, making it difficult for people to contact loved ones.
The OCHA said the signal was detected on the isolated islands of Fonoi and Mango in the Ha'apai Group of islands.
Sixty-nine people live on Fonoi, while 36 live on Mango.
Reconnaissance flights from New Zealand and Australia identified the islands as having "substantial property damage" on the Ha'apai Group, according to OCHA.
British woman Angela Glover was named as the first fatality of the natural disaster.
The 50-year-old, who ran a dog rescue, was swept away in the tsunami which followed the volcanic eruption.