The eruption of Kilauea volcano has intensified and an aviation red alert means a volcanic eruption is underway that could spew ash along aircraft routes, the U.S Geological Survey (USGS) has said.

A red alert means a major eruption is imminent and is “very hazardous for aviation”.

The change in aviation alert level has gone from orange to red.

The warning from the USGS means a major volcano eruption is “imminent, under way or suspected with hazardous conditions both on the ground and in the air.”

Geologists have warned Kilauea’s summit could have an explosive steam eruption which would hurl huge rocks and ash kilometres into the sky.

USGS issued a warning on Tuesday evening: “As of early this morning, eruption of ash from the Overlook vent within Halemaumau crater at Kilauea Volcano’s summit has generally increased in intensity.

“Ash has been rising nearly continuously from the vent and drifting downwind to the southwest.

“Ashfall and vog (volcanic air pollution) has been reported in Pahala, about 18 miles downwind.”

Ash is a new hazard for residents of Hawaii’s Big Island, as they have already been dealing with volcanic gas and lava which has destroyed 37 homes and other structures.

The ongoing eruption has forced the evacuation of about 2,000 residents.

Steve Brantley, a deputy scientist at the Hawaiian Volcano Observatory (HVO), said: “We’re observing more or less continuous emission of ash now with intermittent, more energetic ash bursts or plumes .

“At any time, activity may become more explosive, increasing the intensity of ash production and producing ballistic projectiles near the vent.”

A new fissure has opened in the Lanipuna Gardens subdivision near Leilani Estates, on Tuesday.

The volcanic vent is spewing lava and toxic gases which pushed air quality into “condition red” which can causes “choking and inability to breathe.”

A total of 21 fissures have opened up in Lanipuna Gardens and Leilani Estates and fissure 17 has been shooting lava like a fountain and sending “spatter more than 100 feet into the air.”

The USGS said a phreatic eruption could happen at a crater at the top of the Kilauea volcano.

A phreatic eruption is a stream-driven explosion which happens when water beneath the ground or on the surface is heated by magma, lava, hot rocks or new volcanic deposits.

However phreatic eruptions are hard to forecast and can occur with little or no warning.