Ashfall and air pollution associated with a volcanic eruption have been reported as far as 18 miles downwind of Kilauea in Pahala.
A red warning means a "major volcanic eruption is imminent, underway or suspected with hazardous activity both on the ground and in the air," the USGS said.
"At any time, activity may become more explosive, increasing the intensity of ash production and producing ballistic projectiles near the vent," the agency tweeted.
The eruption on Kilauea began more than a week ago and has forced thousands of people from their homes, destroyed nearly 40 structures, including dozens of homes, and created 18 fissures in the ground surrounding the volcano.
Lava has inundated almost 120 acres and damaged 50 utility poles.
Hawaii authorities are warning residents in areas around the volcano to be prepared to evacuate with little or no advanced warning.
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