New Zealand officials have raised the alert code for the offshore volcano Whakaari/White Island, which is emitting large steam and gas plumes containing more ash.

The island, situated off the east coast of the North Island, erupted suddenly in December 2019, killing 22 people, mostly tourists, who were on it at the time. Tourists are no longer able to visit.

Volcanic ash has been identified more frequently in the plume and evidence of intermittent explosive activity has also been observed during recent observation flights, natural hazards monitoring agency GeoNet said in a statement on Thursday in Wellington. The volcanic alert level is raised to 3 from 2 suggesting eruption hazards near the vent.

Images from both webcams on the mainland and satellites continue to record an increase in volcanic ash in plume, which indicate that there has been a gradual increase in eruptive activity, GeoNet said.

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“During recent observation flights, we have observed impact craters on the main crater floor, indicating that on occasions more explosive activity has occurred, ejecting rocks several hundreds of meters away from the vent,” it said.

GeoNet said its aviation alert code remains at orange, the second-highest level behind red, which cautions of significant emission of ash.