A photo released by the New Zealand Defence Force showed a part of Tonga blanketed in ash
New Zealand is sending aid to disaster-struck Tonga, but ash on the capital's main airport runway is preventing relief planes from landing.
It will also take days before military ships with supplies will reach the islands, said New Zealand authorities.
Fears of a possible humanitarian crisis developing on Tonga are growing after Saturday's massive volcano eruption.
It sparked a tsunami and severed an undersea cable, cutting off the country from the outside world.
New Zealand and Australia have conducted surveillance flights to assess the damage. At least one person is confirmed to be dead, while the number of those injured is still unknown.
On Tuesday New Zealand foreign affairs minister Nanaia Mahuta said that "water is among the highest priorities for Tonga at this stage". Aid agencies say it is likely that volcanic dust and the tsunami had contaminated Tonga's water supplies.
She said a C-130 Hercules aircraft was on standby to fly to Tonga's capital Nuku'alofa to deliver humanitarian aid including collapsible water containers, generators and hygiene kits.
"However images show ashfall on the Nuku'alofa airport runway that must be cleared before (the plane) can land," she said.
On Monday about 200 Tongans had started sweeping the runway yesterday, successfully clearing a 100m (330ft) stretch of tarmac, but there remained "a long way to go", according to 1News reporter Barbara Dreaver.
Ms Mahuta also said two navy ships carrying water supplies, disaster relief stores, and a rescue helicopter will be dispatched to Tonga. However the ships are expected to take three days to arrive.
New Zealand's Acting High Commissioner to Tonga, Peter Lund, said the local government had declared a state of emergency and that the extent of the destruction was starting to come to light.
"[There is] damage to buildings, there's a lot of rubble and rocks that have been thrown up but people now trying to get back to normal in the capital," he told 1News, adding that clean-up efforts were under way to get rid of the "thick film of ash" blanketing the area.
Mr Lund also said three deaths have been reported, but that these were unconfirmed. It is also unclear whether the number includes the one confirmed death of 50-year-old British national Angela Glover, who died as she was washed away while trying to save her dogs.
A distress signal has been detected from two small, isolated Tongan islands, according to the UN.
But the Red Cross has said reports suggested the overall damage was not as bad as had been feared.
"We believe that from the information that we can put together that it is not as catastrophic in those major population centres as we first thought that it might be, so that's really good news," said Katie Greenwood, who is co-ordinating the organisation's response from Fiji.
Communications with the island chain remain extremely limited, making it difficult to establish the scale of the destruction.
Tongans living abroad have been anxiously waiting for news from relatives and loved ones back home, as reports say it may take up to two weeks to restore phone and internet lines.
The Red Cross said even satellite phones, used by many aid agencies, had poor service due to the effects of the ash cloud. The organisation estimates that up to 80,000 people may have been affected by the tsunami.
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