Volcano eruption in New Zealand.

At least one person has been critically injured and an unknown number are missing after a volcano on New Zealand’s popular White Island tourist destination erupted Monday.

New Zealand police said in a statement that fewer than 50 people were believed to be on the island at the time of the eruption, but not everyone had been accounted for.

“Of those transported to shore, at least one has been critically injured. Emergency services are working to ensure the safety of everyone involved, including rescue staff,” the police statement said.

About 20 tourists from the nearby cruise ship Ovation of the Seas were on White Island at the time of the eruption, New Zealand Cruise Association chief executive officer Kevin O’Sullivan told CNN.

“Our hope is that everyone will be recovered quickly and unharmed,” he said. The Ovation of The Seas is one of the largest cruise vessels in the world, according to operators Royal Caribbean.

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Images of the crater from cameras run by the Institute of Geological and Nuclear Sciences appear to show a group of people inside the smoking center of the volcano just minutes before the eruption.

Speaking at a press conference, Prime Minister Jacinda Ardern said that search and rescue crews were assisting at the site of the eruption, which is located off the eastern coast of the North Island.

Originally she said there had been around 100 on the island but police later said the estimate was now lower.

Ardern said it was too early to say how bad the injuries were but added it was her understanding that a number of those affected were tourists.

Speaking to CNN-affiliate Radio New Zealand, St. John Ambulance said that up to 20 people are believed to have been injured in the eruption, adding that a mobile triage unit was on its way.

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Photos from New Zealand’s official geological hazard information site Geonet showed a huge plume of white smoke rising above the island on Monday afternoon, local time.

The White Island or Whakaari volcano is New Zealand’s most active cone volcano according to the GeoNet website. It has been built up by more than 150,000 years of volcanic activity.

A cone volcano is the most immediately recognizable mountain-shaped variety, as opposed to shield volcanoes or calderas which are far more flat in shape.

A volcanic ash advisory was issued at 2:30 p.m. local time (8:30 p.m. ET Sunday) by MetService, New Zealand’s meteorological service.

New Zealand Police called for people in the affected ashfall areas to stay indoors.

The Volcanic Alert Level was raised to 4 shortly after the eruption, and the Aviation Color code is orange, according to GeoNet. Two hours later it was lowered to level 3, due to “diminished” activity in the area of the volcano.

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New Zealand’s National Emergency Management Agency said the eruption occurring at White Island is “hazardous in the immediate vicinity of the volcano” and that the agency is assessing the situation with scientific advisors to determine the severity of the threat.

Whakaari/White Island is dubbed “one of the world’s most accessible active volcanoes” on a White Island Tours website, which offers ocean cruises near the island and a guided tour depending on the status of the varying volcanic alert levels.

According to the GeoNet website, more than 10,000 people visit the island every year.