Search crews in New Zealand recovered six of eight bodies Friday from an island where a volcano erupted, killing up to 16 people, authorities said.

The recovery operation at White Island will continue while two people presumed to be dead are unaccounted for, New Zealand police said in a statement. They said divers were looking for one body that was seen in the water.

"We are making every effort to locate and recover the two remaining deceased," police said.

Six men and two women from New Zealand's military and police forces went onto White Island on Friday, despite the risk of another eruption. New Zealand Police Commissioner Mike Bush said the operation "went to plan."

The geological agency GeoNet said Friday that although the level of volcanic tremors has dropped, it remains very high compared to pre-eruption levels, and steam and mud bursts were continuing.

Before Friday, eight people had been confirmed dead, including two who died in hospitals following Monday's surprise eruption. Many survivors suffered severe burns, officials have said.

Police said the search team faced a highly unpredictable and challenging environment.

"They showed absolute courage and commitment to ensure we can offer some closure to the families and friends of those who have lost loved ones," police said in the statement.

White Island is roughly 30 miles off the coast of New Zealand's North Island and is known as a stratovolcano, characterized by continuous small-to-moderate eruptions over the past 150,000 years. Scientists say the volcano is difficult to predict, making it one of the most dangerous in the world.

About 47 tourists, including nine Americans, were on the island when the volcano erupted.

New Zealand police said they intend to open an investigation into surrounding events, including an inquest into health and safety issues for tourists and tour companies.

White Island is privately owned but became part of a scenic reserve in 1953, and daily tours bring more than 10,000 visitors each year, according to GeoNet's website. Around 70 percent of the volcano is underwater, and it is the largest volcanic structure in the country.

New Zealand Prime Minister Jacinda Ardern on Friday thanked those involved in the recovery and said her thoughts were with the victims' families.

"We know that reunification won't ease that sense of loss or grief because I don't think anything can," Ardern said in a statement. "But we felt an enormous duty of care as New Zealanders to make sure that we brought their family members back."