A volcano has erupted in New Zealand, leaving five people dead and several unaccounted for, police have said.
Tourists were seen walking inside the crater of White Island volcano moments before Monday's eruption.
Twenty-three people have been rescued, but it is unclear how many people remained on the island. Police say aerial reconnaissance flights have not identified any sign of life there.
White Island, also called Whakaari, is the country's most active volcano.
Despite that, the privately owned island is a tourist destination with frequent day tours and scenic flights available.
Police have said fewer than 50 people were on the island when the volcano erupted. Deputy Police Commissioner John Tims told reporters that "both New Zealand and overseas tourists" were believed to be involved.
Rescuers are not able to reach the island because of the risk of further eruptions, he added. It is currently night time in the area.
Some of those injured are in a critical condition, Radio New Zealand reported.
What happened at the volcano?
The eruption of White Island began at about 14:11 local time (01:11 GMT).
Visitor Michael Schade - who was on a boat leaving the island after a morning tour - filmed a thick plume of ash and smoke as the volcano erupted.
He told the BBC he was at the crater just 30 minutes before the eruption.
"It was still safe-ish but they were trying to limit the group sizes [of people visiting the volcano]."
Describing the eruption, he said: "We had just got on the boat... then someone pointed it out and we saw it. I was basically just shocked. The boat turned back and we grabbed some people that were waiting on the pier."
Another witness, Brazilian Allessandro Kauffmann, narrowly missed the eruption.
"There were two tours that went to this volcano today. One of them was ours, which was the first. We left five minutes before the volcano erupted," he posted on Instagram in Portuguese.
"This other tour that arrived right after, unfortunately they did not manage to leave in time, and there were some people that suffered serious burns."
A live feed from the volcano showed a group of visitors inside the crater before images went dark.
Who was on the island?
New Zealand Prime Minister Jacinda Ardern said: "I know there will be a huge amount of concern and anxiety for those who have loved ones at the island at that time - and I can assure them police are doing everything they can."
She said falling ash was hampering attempts by rescuers to get to the site.
The New Zealand Defence Force is now helping the rescue operation. A military plane has carried out surveillance and two helicopters and personnel are ready to assist.
Police initially said there were 100 people on or near the island, but later revised down the number to 50.
Some of them were passengers from the Ovation of the Seas, a cruise ship owned by Royal Caribbean. It is currently at port in Tauranga, a coastal city near White Island.
Australian Prime Minister Scott Morrison said Australians had "been caught up in this terrible event", adding that authorities were "working to determine their wellbeing".
Was there any forewarning of an eruption?
On 3 December, geological hazard monitoring website GeoNet warned "the volcano may be entering a period where eruptive activity is more likely than normal", although it added "the current level of activity does not pose a direct hazard to visitors".
University of Auckland associate professor Jan Lindsay said the alert level was recently raised from one to two. "There was a heightened level of unrest and everyone was aware," she said.
"[The volcano] has a persistently active hydrothermal system... if gases build up under a block of clay or mud they can be released quite suddenly," Prof Lindsay said.
"It's possible that there's no magma involved, that it's just a phreatic eruption - a steam eruption. We don't know yet."
When asked if visitors should have been on the island, Prof Lindsay said: "It's a difficult question. It's often in a state of heightened unrest.
"It's a privately owned island and with lots of private tour operators. It is not part of the conservation estate - and so not under government control.
"GNS [New Zealand's geoscience institute] put out their alert bulletins and have good communication with tour companies, and they know what the risk is. "
White Island has seen several eruptions over the years, most recently in 2016, but no-one was hurt.
Seismologist Ken Gledhill said: "It was kind of almost like a throat-clearing kind of eruption - and that's why material probably won't have made it to mainland New Zealand.
"It went up about 12,000 metres into the sky and so... on the scheme of things for volcanic eruptions it's not large, but if you were close to that, it is not good."
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