PAHOA, Hawaii (Reuters) - Explosions intensified on Hawaiis Kilauea volcano on Tuesday, spewing ash and triggering a red alert for aircraft for the first time since the latest eruption began 12 days ago.

Ash and volcanic smog, or vog, as it is called, rose to 12,000 feet (3,657 meters) above Kilaueas crater and floated southwest, showering cars on Highway 11 with gray dust and prompting an unhealthy air advisory in the community of Pahala, 18 miles (29 km) from the summit.

An aviation red alert means a volcanic eruption is under way that could spew ash along aircraft routes, the U.S. Geological Survey (USGS) says on its website.

Ash was also a new hazard for residents of Hawaiis Big Island, already grappling with volcanic gas and lava that has destroyed 37 homes and other structures 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 (NOAA).

Were observing more or less continuous emission of ash now with intermittent, more energetic ash bursts or plumes, Steve Brantley, a deputy scientist in charge at the Hawaiian Volcano Observatory (HVO), said on a conference call with reporters.

The observatory warned the eruption could become more violent.