New Zealand has sent a plane to Tonga to assess the damage after a huge volcanic eruption triggered a tsunami.
The eruption has covered the Pacific islands in ash, cut power and severed communications.
Up to 80,000 people there could be affected, the International Federation of Red Cross and Red Crescent Societies (IFRC) told the BBC.
New Zealand Prime Minister Jacinda Ardern said the tsunami had wreaked "significant damage".
No deaths have been reported so far.
Information remains scarce, however, and New Zealand and Australia are sending surveillance flights to assess the extent of the damage.
The New Zealand Defence Force tweeted that an aircraft had left to "assist in an initial impact assessment of the area and low-lying islands".
Katie Greenwood of the IFRC in Fiji said that help was urgently needed.
"We suspect there could be up to 80,000 people throughout Tonga affected by either the eruption itself or from the tsunami wave and inundation as a result of the eruption," she said.
"That was a shock to people, so we do hold some concern for those outer islands and we're very keen to hear from people."
The underwater volcano erupted on Saturday, sending a plume of ash into the sky and triggering warnings of 1.2m (4ft) waves reaching Tonga. The eruption was so loud it could be heard in New Zealand, some 2,383km (1,481 miles) from Tonga.
New Zealand's Acting High Commissioner in Tonga Peter Lund has said the island nation looks "like a moonscape" after it was coated in a layer of volcanic ash.
The dust was reportedly contaminating water supplies and making fresh water a vital need, Ms Ardern said on Sunday.
Aid charities said the ash had prompted authorities to tell people to drink bottled water and wear face masks to protect their lungs.
As the sky darkened with ash, videos showed traffic jams as people fled low-lying areas by car. Hours later, Tonga's internet and phone lines went down, making the island's 105,000 residents almost entirely unreachable.
Prior to the largest eruption, the volcano had been erupting for several days. The Tonga Meteorological Agency had warned that the smell of sulphur and ammonia was being reported in some areas.
Ms Ardern said power was being restored to some parts of the island and mobile phones were slowly starting to work again. But the situation in some coastal areas remained unknown.
Unable to speak to their friends and family, many Tongans in Australia and New Zealand have grown concerned for their safety.
Fatima said she had not heard anything from her colleague who runs a seafront restaurant in Tonga's capital Nuku'alofa.
"It's all very sad, we are hoping for the best," she told the BBC. "This will hit them so hard as they have been in lockdown a long time with no tourists visiting and now this."
Satellite images suggest some outlying islands have been completely submerged by seawater.
Experts say the eruption of the Hunga-Tonga Hunga-Ha'apai volcano is one of the most violent in the region in decades.
It triggered tsunami warnings in several countries, including Japan and the US, where flooding hit some coastal parts of California and Alaska.
UK Foreign Office minister Zac Goldsmith called the situation in Tonga "shocking" and said Britain stood "ready to help and support our Commonwealth friend and partner in any way we can".
Satellite images offer glimpse at volcano's destruction
Scientists got their first look at Hunga-Tonga Hunga-Ha'apai post-eruption on Saturday when the EU's Sentinel-1A satellite flew overhead.
This spacecraft is a radar platform and can see through obscuring cloud and ash to the surface below.
It showed clearly that much of the crater rim that stood above the ocean waters had been destroyed - a testament to the ferocity of the blast.
Researchers will be keen to understand the cause of a tsunami that produced waves not just on nearby islands but at beaches right around the Pacific.
To what extent was the explosion itself responsible? Pressure waves can make what are termed "meteotsunami". Or perhaps the displacement of water was the result of an unseen collapse of part of the volcano below the ocean surface.
More data in the coming days and weeks will establish the facts.
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