A plume of smoke can be seen rising from Hunga-Tonga-Hunga-Ha'apai volcano in images taken before the eruption, with the volcanic island appearing to have all but disappeared afterwards.

Australia and New Zealand sent military surveillance flights to assess damage to the Pacific island nation on Monday after a towering ash cloud following Saturday's eruption prevented earlier flights.

UN humanitarian officials and Tonga's government have reported "significant infrastructural damage around Tongatapu", said UN spokesman Stephane Dujarric.

"There has been no contact from the Ha'apai Group of islands, and we are particularly concerned about two small low-lying islands - Mango and Fonoi - following surveillance flights confirming substantial property damage," Mr Dujarric said.

According to Tonga's government, 36 people live on Mango and 69 on Fonoi.

Tonga tsunami: Body of British woman swept away by wave found

New Zealand's High Commission in Tonga also reported "significant damage" along the western coast of the main island of Tongatapu, where there are several holiday resorts.

New Zealand's military has sent aid deliveries of drinking water and other supplies but these have been delayed by thick ash on an airport runway. The country is also sending two navy ships that will leave on Tuesday and has pledged an initial 1 million New Zealand dollars (£498,000) towards recovery efforts.

The UN World Food Program is also exploring how to bring in relief supplies, though one complicating issue is that Tonga has managed to avoid outbreaks of COVID-19. New Zealand has said its military staff are vaccinated and willing to follow Tonga's protocols.

Communications with the island have been limited after the eruption cut the islands main undersea communications cable.

Distress signal detected on isolated Tonga islands

New Zealand's Acting High Commissioner for Tonga, Peter Lund, has said there were unconfirmed reports of up to three fatalities on Tonga so far.

One death has been confirmed, with the family of British woman Angela Glover, 50, saying she was swept away by a wave.

Nick Eleini said his sister's body had been found and that her husband survived.

"I understand that this terrible accident came about as they tried to rescue their dogs," Mr Eleini told Sky News.