A volcanic eruption on New Zealand's White Island has killed five people, but the death toll could rise again once rescuers carry out a thorough search for bodies.

Several people were missing and up to 20 suffered injuries, including serious burns, after visitors walking inside the crater were caught up in the explosion.

Harrowing video shows hikers, including foreign tourists, scrambling for cover and later waiting to be rescued as the volcano spews debris and a plume of ash and smoke 12,000ft into the air.

The entire island is covered in ash and volcanic material and there are fears of additional eruptions, making it too dangerous for police and rescuers to set foot on the island.

At least 23 people - some of them believed to be tourists from the Ovation of the Seas cruise ship - were rescued following the explosion just after 2 pm local time on Monday.

Harrowing video shows tourists scrambling for cover as the volcano spews a plume of ash and smoke 12,000ft into the air.

The entire island is covered in ash and volcanic material and there are fears of additional eruptions, making it too dangerous for police and rescuers to set foot on the island.

The five people who died had all been rescued from the island with severe injuries. Many of those who were rescued had suffered burns, said officials.

A crater rim camera owned and operated by New Zealand science agency GeoNet shows groups of people walking toward and away from the rim inside the crater, from which white vapour constantly billows, in the hour leading up to the eruption.

At 2 pm the crater rim camera catches a group of people - tiny specks in relation to the vast volcano - right at the edge of the rim.

At 2.10 pm - just a minute before the eruption - the group is headed away from the rim, following a well-worn track across the crater.

Police said less than 50 people were on the island, which is about 30 miles off the coast of New Zealand's North Island and is home to the country's most active volcano.

It is understood that about half of them were from Australia.

New Zealand's national police force said: "There remain a number of people on the island who are unaccounted for.

"Both New Zealanders and overseas tourists are believed to be involved. At this stage, we can confirm there has been one fatality, and based on the information we have it is likely there are others. Police are leading the search and rescue operation and we are working at a pace to confirm the numbers of those involved.

The island is currently covered in ash and we are taking expert advice with regards to the conditions to determine when we can safely access the island."

National Operation Commander Deputy Commissioner John Tims told reporters at a press conference: "At this stage, it is too dangerous for police and rescue to go onto the island.

"The island is currently covered in ash and volcanic material."

Prime Minister Jacinda Ardern confirmed tourists were on the island at the time. She said: "I know that there will be a huge amount of anxiety for those who had loved ones on the island at the time."

In one harrowing video clip posted online, panicked holidaymakers on a tour boat just off the island are rushed inside as smoke billows towards them.

A worker says over the tannoy: "Everybody inside the boat now."

A man is overheard saying "We've got to get out of here" while another repeatedly shouts "go inside".

There are fears that the volcano could erupt again, adding to the danger for rescuers.

Dr Ken Gledhill, of the geoscience agency GNS, said: "It was not a particularly big eruption, it was almost like a throat-clearing eruption and that's why material probably won't make it to mainland New Zealand. We can't be certain it won't erupt again in the next 24 hours."

Some of those who were on the island were from the Ovation of the Seas cruise ship, which left Sydney last Wednesday on a 12-day tour.

Kevin O'Sullivan, chief executive of the New Zealand Cruise Association, said: "We believe there is a tour party from Ovation of the Seas involved in the White Island eruption. We have no further details at the moment."

Ovation of the Seas is a 16-deck cruise ship owned by Royal Caribbean Cruises. It can take nearly 5,000 passengers and has a crew of about 1,500. It is docked at Tauranga, on the North Island.

The company spokesperson said: "We can confirm that a number of our guests were touring the island today.

"We do not have any additional details to share at this time."

A no-fly zone has been set up around the island.

Judy Turner, the mayor of the nearby coastal town of Whakatane, said: "I'm not sure if these people were on the island or near it, but there was definitely one group out there and they definitely needed medical care. There were some injuries and the focus is on getting these injured people back safely and to get them to a hospital."

There seemed to be no danger for people in coastal areas farther away, she added.

Calvin Kingi, whose Facebook profile says he works at White Island Tours, posted a photo of the eruption, saying: "White Island just erupted as we left, we have our workmates and a tour still on the island, I hope they ok."

He added that people were in a "critical condition".

Around 10,000 people a year visit the volcano, which is also known by its M?ori name Whakaari.

Last month, GNS raised the volcano's alert level to two - meaning "moderate to heightened volcanic unrest" - because of increasing inactivity.

GNS warned: "Hazards on the island are now greater than during the past few weeks.

"The patterns of signals are similar to those through the 2011-2016 period and suggest that Whakaari/White Island may be entering a period where eruptive activity is more likely than normal."

The alert level was raised to four or "moderate volcanic eruption", the second-highest level, after Monday's explosion.

It was later downgraded to three, meaning "minor volcanic eruption".

Monday's eruption was unexpected, but not unusual, an expert said. "Sudden, unheralded eruptions from volcanoes such as White Island can be expected at any time," Shane Cronin, a volcanologist at the University of Auckland, said in comments published by the Australia Science Media Centre.

"We know hydrothermal and so-called 'phreatic' eruptions can occur suddenly and with little or no warning because they are driven by the expansion of super-heated water into steam," he added.

During a period of increased activity between 2011 and 2016, experts warned people not to venture onto White Island. Scientists had been warning for years that the volcano could erupt at any time. But that hasn't stopped holidaymakers and guides from visiting the site.