USGS is warning there could still be an “explosive eruption” of the Kilauea Hawaii volcano, last seen in 1924.
The US scientific agency said pent-up steam could drive a 20,000-foot (6,100-meter) ash plume out of the crater.
This would spread debris over a 12 mile (19 km) radius.
Ash and volcanic smog spewing from the volcano also triggered a red alert for aircraft for the first time since the latest eruption began 12 days ago.
According to the U.S. Geological Survey (USGS), a red alert means a major eruption is imminent or underway and ash could affect air traffic.
Hawaii County officials have warned of toxic gas and urged residents to leave the area as it may cause suffocation.
They said: “Severe conditions may exist such as choking and inability to breathe.
“This is a serious situation that affects the entire exposed population.”
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.
Steve Brantley, a deputy scientist in charge 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.”
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 in a statement on the change in aviation alert level to red from orange.
The area taking the brunt of the eruption is about 25 miles (40 km) down Kilauea’s eastern flank, near the village of Pahoa. Lava has burst from the ground to tear through housing developments and farmland, threatening one of the last exit routes from coastal areas, state Highway 132.
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,” the Hawaiian Volcano Observatory and Hawaii County Civil Defense said.
Summer hotel bookings on Big Island have dropped by almost half from last year, according to the Island of Hawaii Visitor Bureau.
One visitor who had planned to visit Hawaii Volcanoes National Park before it was shut was exchange student Constantin Plinke.
The 24-year-old said: ”We had a big list of things to do and maybe 80 percent of them were in the national park. It’s sad.”