A volcano has erupted in New Zealand, leaving five dead and several unaccounted for, police have said.

Tourists were seen walking inside the rim of the White Island crater just moments before the eruption.

Police said 23 people had been rescued, but warned conditions were hampering rescue efforts. The New Zealand military is now helping police.

White Island, also known as Whakaari, is one of the country';s most active volcanoes.

Despite that, the privately-owned island is a tourist destination with frequent day tours and scenic flights available.

What happened at the volcano?

The eruption of White Island began around 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 there were "a number" of tourists on or around the island, both from New Zealand and overseas.

"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.

She said police had launched a search and rescue operation but falling ash was hampering attempts to get to the site.

The New Zealand Defence Force is now helping the rescue operation, working under the police. A military plane has carried out surveillance and two helicopters and "personnel" are ready to assist.

Deputy police commissioner John Tims had earlier said "it was too dangerous at this stage for police and rescue services to go to the island".

Police initially said there were 100 people on or near the island, then later revised down the number to 50.

Some of the passengers are from cruise ship Ovation of the Seas, 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 the eruption expected?

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 they added "the current level of activity does not pose a direct hazard to visitors".

Associate Professor at the University of Auckland, 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.

"It [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 added.

"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 Linsday 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.

Police are warning people living near the area to "be aware of the potential for ashfall" and to stay indoors.

The eruption is not expected to affect North Island.

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 twelve-thousand 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."