PAHOA, Hawaii - Explosions have intensified on Hawaii's Kilauea volcano, spewing ash and triggering a red alert for aircraft for the first time since the latest eruption began 13 days ago.

Ash and volcanic smog, or vog, as it is called, rose to 3,660 meters on Tuesday above Kilauea's crater and floated southwest, showering cars on Highway 11 with gray dust and prompting an "unhealthy air" advisory in the community of Pahala, nearly 30 kilometers from the summit.

An aviation red alert means a volcanic eruption is under way that could spew ash along aircraft routes, the US Geological Survey said.

Ash was also a new hazard for residents of Hawaii's Big Island, already grappling with volcanic gas and lava that has destroyed 37 homes and forced the evacuation of about 2,000 residents.

A shift in winds was expected to bring ash and vog inland on Wednesday and make them more concentrated, said John Bravender of the National Oceanic and Atmospheric Administration.

"We're observing more or less continuous emission of ash now with intermittent, more energetic ash bursts or plumes," said Steve Brantley, a deputy scientist in charge at the Hawaiian Volcano Observatory.

The observatory warned the eruption could become more violent.

"At any time, activity may become more explosive, increasing the intensity of ash production and producing ballistic projectiles near the vent," the HVO said on the change in aviation alert level to red from orange.

Ash is not poisonous but irritates the nose, eyes and airways. It can make roads slippery and large emissions could cause the failure of electrical power lines, said USGS chemist David Damby.

The eruption has hit the island's tourism industry.

Big Island summer hotel bookings have dropped by almost half from last year, said Rob Birch, executive director of the Island of Hawaii Visitor Bureau.

College exchange student Constantin Plinke, 24, was planning to go to the Hawaii Volcanoes National Park before it was shut.

"We had a big list of things to do and maybe 80 percent of them were in the national park," he said, after stopping by the side of the road to watch ash plumes rising into the air. "It's sad".

The latest fissure in the earth opened on Tuesday, spewing lava and toxic gases that pushed air quality into "condition red" around Lanipuna Gardens and nearby farms, causing "choking and inability to breathe," authorities said.

Reuters - Ap