New Zealand's White Island volcano is showing increased seismic activity, putting recovery efforts on hold.
Authorities had hoped they would be able to retrieve the bodies of those left behind on the island, all of whom are presumed dead, on Wednesday.
The exact death toll is unclear. Police say at least six people died, two of them since leaving the island, and nine are officially missing.
Another 22 people are in intensive care with severe burns.
One has been moved to Australia.
The volcano, also known as Whakaari, erupted on Monday when dozens of tourists were on the island.
"I've spoken to many of those involved in the operation and they are very, very eager to get back there, they want to bring people's loved ones home," Prime Minister Jacinda Ardern told reporters.
What condition are the injured in?
White Island is a popular tourist destination with frequent day tours and scenic flights available. At least 47 visitors from around the world were on the volcano crater at the time of two explosions in quick succession.
Police originally said that of the 30 injured, 25 were in a critical condition while the other five were classed as stable but serious.
Mr Nash explained the injuries to the survivors were so severe that some of them were unable to identify themselves.
"There are a number of people in hospital who cannot communicate, they have significant burns not only to skin but internal organs," he told Radio New Zealand.
"We wish them the best but we're not out of the woods yet, of that there's no doubt."
Dr Peter Watson, chief medical officer at New Zealand's National Burns Unit, said an estimated 1.2m sq cm of replacement skin would be needed for the patients. An order has been placed from the US.
Several patients would be transferred to Australia by the Australian Defence Force using an intensive-care acceptable aircraft in the next 24-48 hours, Dr Watson said.
Where does donor skin come from?
Analysis by James Gallagher, BBC health and science correspondent
Donated skin is vital for helping to save the life of a major burns patient. Skin is our largest organ and its main job is keeping dangerous viruses and bacteria out.
The damage caused by severe burns leaves patients at very high risk of infection. Donor skin is a short-term fix.
It comes from dead organ donors - in the same way as hearts, kidneys and corneas - and can be banked for several years.
The donor-skin aids healing, cuts the risk of infection and can reduce pain. The immune system is so weak after such an injury that rejection is not an issue.
A single major burns patient needs a lot of donor skin. New Zealand is treating many at the same time and has needed to turn to other countries for help.
What do we know about the victims?
New Zealand's chief coroner on Wednesday declared the eruption a "mass fatality incident".
Officials said they are working with disaster specialists and forensic experts to identify the victims so their bodies can be returned to their families.
New Zealand police have now listed nine people as officially missing - though they say this is a partial list as they have not been able to speak to all the next of kin.
- Hayden Inman (New Zealand)
- Tipene Maangi (New Zealand)
- Julie Richards (Australia)
- Jessica Richards (Australia)
- Gavin Dallow (Australia)
- Zoe Hosking (Australia)
- Richard Elzer (Australia)
- Karla Matthews (Australia)
- Krystal Browitt (Australia)
At least seven people on the list are believed to be dead or presumed dead, based on relatives talking to media.
Tour guide Hayden Inman was identified as among the dead by his brother on Facebook.
Julie Richards and her daughter Jessica from Brisbane, Australia, were identified as victims by a family spokesperson.
Adelaide father Gavin Dallow has been named as dead, with his stepdaughter Zoe Hosking presumed dead, according to media reports.
Australian tourist Jason Griffiths, who was travelling with Karla Matthews and Richard Elzer, died in hospital on Wednesday. According to a statement released by their friends, both Karla and Richard are presumed dead.
There is also a definitive list of all victims who are in hospital but police say they cannot release this for privacy reasons.
Why is the recovery delayed?
On Wednesday morning, geological agency GeoNet said "the level of volcanic tremor has significantly increased at the island".
Scientists view tremors, which result from a release of energy under the Earth's surface, as one sign of a possible eruption.
In a later update, the agency said volcanic tremor, known as seismic activity, was now at the highest level seen since 2016.
"The level of volcanic tremor continues to rise and there is medium likelihood of future eruptive activity in the next 24 hours," the agency said.
At a news conference on Wednesday, volcanologist Graham Leonard said seismic activity on White Island was escalating.
"Yesterday there was a high risk of an eruption," Mr Leonard said. "Today there is an even higher risk of an eruption. And the parameters are worsening at the moment."
Police said the latest seismology update meant recovery teams had no choice but to wait, but they were on constant standby to return to the island as soon as possible.
Police Minister Nash said there were also poisonous gases coming from the volcanic vent and that the island was blanketed in a thick layer of acidic ash.
With measuring equipment on the island still intact, GeoNet can give regular updates on the situation, allowing police to assess the risk of sending recovery teams.
Reconnaissance flights have shown no signs of life on the island and officials believe there are no survivors among the missing.
Police said a drone conducted four fly-overs of the island on Wednesday, and analysis of that footage was ongoing.
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