With communications down, a ship made it to Nomuka, Mango and Fonoifua on Wednesday, an official said.

Few homes are said to remain standing after settlements were hit by 15m (49ft) waves.

"Very unfortunate information has come to light overnight about the three islands that we were really worried about," the Red Cross's Katie Greenwood said.

"They have all suffered devastating consequences as an effect of these incoming waves. Most of the structures and dwellings on those islands have been completely destroyed."

Images believed to be from the village of Kanokupolu, on the island of Tongatapu, show trees down and debris strewn across the ground.

Radio broadcaster Marian Kupu, speaking from the capital of Tonga, Nuku'alofa, also on Tongatapu, told Sky News the eruption had been "like a bomb".

She added: "Everything on the island - houses, cars - it's all grey from the dust."

People are being advised to stay in their homes and close all windows and doors, with homes and communities completely coated in ash. You can see the impact in the before and after picture below.

Power is coming and going, and clean drinking water is needed soon, Ms Kupu said.

Two New Zealand navy vessels are due to arrive in Tonga on Friday, carrying water supplies.

"Securing access to safe drinking water is a critical immediate priority as there is a mounting risk of diseases such as cholera and diarrhoea," Ms Greenwood said.

At least three people have died in the disaster, including British woman Angela Glover.

At least a month to restore communication cable

Meanwhile, an undersea communication cable connecting Tonga to the rest of the world will take at least a month to fix, its owner said on Wednesday.

"The cables are actually around the volcanic zone," said Samiuela Fonua, chairman of Tonga Cable Ltd. "We don't know whether they are intact or blown away or stuck somewhere underwater. We don't know if it's buried even deeper."

A specialist ship is intending to embark from Port Moresby, the capital of Papua New Guinea, at the weekend, Mr Fonua added.

But it will take eight or nine days to reach Samoa to collect equipment, and it will be "lucky" if the job is done
within a month.

"It could be longer than that," Mr Fonua said.

More world news:
Second man arrested in Ashling Murphy murder probe
Inside the eerie village on frontline of Russia-Ukraine tensions

International phone service restored

Telecoms operator Digicel said it had restored international phone connectivity, but internet links come via the undersea cable.

Meanwhile, a British man living in Tonga told Sky News the sound of the volcano erupting was "like bombs going off around the place".

Alistair Coldrick, who runs whale-watching tours on the island of Vava'u, said he and his family fled to higher ground after hearing the explosion.