At least five people have died and as many as 20 were injured after a volcano began erupting Monday afternoon off New Zealand's North Island, one of the country's two main islands.

Speaking to reporters in the capital Wellington, New Zealand's Deputy Police Commissioner John Tims confirmed the fatalities and said the toll is likely to rise, since there are still several people missing.

Tim said the police were not sure about the exact number of people still in the island and their wellbeing, but could be in the double digits.

He said they were of different nationalities, and some of those involved were guests from the Royal Caribbean International cruise ship Ovation of the Seas.

Tim also said the island was still too dangerous for police and rescuers to search for the missing. He said that most of the 18 survivors were injured, and some with severe burns. They were transported to the area hospitals for treatment.

Emergency officials say around 50 people were on White Island, also known as Whakaari, when the eruption began, fewer than initially reported by Prime Minister Jacinda Ardern who said 100 tourists were "on or around" the volcano,.

AFP reports that cameras providing a live feed from the volcano showed a group of tourists walking on the crater floor moments before the eruption occurred.

White Island sits 50 kilometers northeast of the town of Tauranga on North Island.

Authorities urged people to avoid areas on North Island near the eruption.

GeoNet agency classified the volcanic eruption as moderate and raised its alert level to four, on a scale where five represents a major eruption.

GeoNet says White Island is New Zealand's most active cone volcano and about 70 percent of the volcano is under the sea.