New Zealand scientists say activity at a volcano on an island to the east of the North Island has led them to upgrade a warning to aircraft and to sightseers.

The White Island volcano is showing an increasing level of unrest, causing the government's Institute of Geological and Nuclear Sciences to raise its aviation alert code Thursday from yellow to orange, indicating an increased likelihood of eruption, China's Xinhua news agency reported.

Explosions and ash emissions began at White Island in August and activity has been increasing since about Jan. 14, scientists said, with increased hydrothermal activity in the island's "hot lake."

"Visitors to White Island are now at the highest level of risk since the start of the 2012 eruptions," GNS vulcanologist Gill Jolly said in a statement.

"Hazards to visitors can include the health effects of volcanic gas exposure, including respiratory issues, skin and eye sensitivity to acid gases," he said. "Explosive eruptions can occur at any time with little or no warning. We advise a high level of caution should be taken if visiting the island."

White Island's current activity is the first since an eruption in 2001.