American tourists were among the eight people still missing Monday after a volcano on an island off the coast of New Zealand erupted, killing at least five people, the country's prime minister said.

Prime Minister Jacinda Ardern said she believes tourists from Australia, China, Malaysia and the United Kingdom are also among the missing and warned they may not find any other survivors on Whakaari White Island, which has been covered by a blanket of ash at least a foot thick.

"We share in your unfathomable grief in this moment and time," Ardern said. "For now, our duty is to return loved ones."

So far, reconnaissance flights over the area in the hours after the eruption revealed no signs of life, Ardern said.

"The focus has to be on those who are...critically injured and, of course, what is now a recovery" mission, Ardern said.

But rescue workers have to dread carefully. "It is a very unpredictable volcano," said Ardern.

Thirty-one people who were rescued from White Island remain hospitalized and three others have already been discharged, the prime minister added.

The country's most active cone volcano, located in the Bay of Plenty about 30 miles off the northeast New Zealand coast, erupted at 2:11 p.m. Monday (8:11 p.m. Sunday ET), according to GeoNet, the government earthquake agency. Ardern clarified that there were in fact two separate eruptions that occurred in quick succession.

"It just looked like what you see of a nuclear bomb going off, is what it looked like, kind of was turning into a mushroom cloud," Dan Harvey, a commercial fisherman who was out at sea at the time of the eruption, told Radio New Zealand. "The way it just expanded around itself and just went straight up into the sky."

Search-and-rescue operations stalled because it was too dangerous to approach the island, John Tims, deputy commissioner of the national police, said.

Boats, ships and emergency aircraft in the area removed 23 people from the island just after the eruption, many of them with burns, Tims said at an earlier news conference. The five who were killed were part of that group, he said. About 50 people were believed to have been in the area at the time of the eruption.

Tims gave few details on the identities of the people who were killed other than to say that they were from a range of countries.

Michael Schade of San Francisco had just left the volcano and was starting to eat lunch on a tour boat when the volcano began to erupt. He described how the crew on the boat quickly got everyone inside and sped away from the dock.

"It went from nothing going on to it erupting," he said.

After a few minutes, the boat turned around to rescue people waiting on the pier. They boarded with a range of injuries, including burns, he said. The crew and passengers gathered water, medicine and clothing to use as blankets and bandages.

"There was one woman in particular that my mom stayed with and she just had a hard time all together staying awake," he said. "For other people, it was just trying to soothe their burns as best you could without making it worse."

The injuries of those rescued ranged from critical and serious to moderate and minor, according to St. John Ambulance Service, which responded on White Island shortly after the eruption with 11 helicopters as well as other rescue vehicles.

Jonathon Fishman, a spokesman for Royal Caribbean Cruises, told NBC News that multiple guests aboard the ship Ovation of the Seas were touring the island, which in quieter times is a tourist attraction popular with birdwatchers.

In the hour before the eruption, a camera owned and operated by GeoNet showed groups of people walking near the rim inside the crater, where white smoke constantly billows at a low level, according to Reuters.

White Island eruption was also visible on satellite images. ^AJ pic.twitter.com/Cx4LYHr9Ie — MetService (@MetService) December 9, 2019

The camera, along with three others from different vantage points, captures and posts images online of the volcano every 10 minutes. At 2:00 p.m. the crater rim camera captured a group of people right at the edge of the rim.

At 2:10 p.m. — just a minute before the eruption — the group is headed away from the rim, following a well-worn track across the crater.

It is unclear whether the group, which appeared to be made up of around a dozen people, had been alerted to flee or were continuing a tour and unaware of the looming eruption.

Schade said that while his group was on the tour of the island, it stopped by the volcano's main crater and stood over it.

"You can kind of walk right up near the edge and look in. Not too close, but look into it and see the steam bubbling up from it," he said.