Redoubt Volcano, which looms some 3,108 meters (10,200 feet) over a sparsely populated area some 150 kilometers (90 miles) from the city of 280,000 people, has been showing more seismic activity than normal over the past five days.

Dave Schneider, a volcanologist at the Alaska Volcano Observatory, told AFP the recent activity pointed to an eruption similar to the last significant blast at the crater in 1989-1990.

"The most likely outcome of the current unrest is an eruption similar to what we saw in 1989-1990," Schneider said.

"For around 18 hours or so we've had some pretty extensive periods of volcanic tremor. The volcano's been shaking constantly and that's what we'd expect to occur before an eruption actually happens."

The observatory said a pilot who scanned the area on Friday afternoon had reported "no sign of ash emission, but observed significant steaming" in the volcano's summit crater.

The observatory has issued an "orange" alert at the site, warning planes of possible dangers if the volcano erupts.

The four engines of a Boeing 747 became clogged in 1989 when the plane flew over the cloud of ash spewed out by Redoubt during its last eruption.

The pilot managed to restart the engines and save all 231 passengers.

An erupting volcano can release plumes of ash several kilometers (miles) high, but the direction of the cloud and whether Anchorage would be threatened depends on the wind.

"Like most eruptions in Alaska, the greatest hazard is from airborne volcanic ash which is tiny bits of broken rock and glass that comes out of the volcano," Schneider said.

"That is hazardous for two reasons: aviation, and a fallout hazard but that is probably more of a nuisance rather than a danger."

A Federal Aviation Administration spokesman said pilots in the region had been advised of the volcano's status.

Schneider said there were no communities in the immediate vicinity of the volcano and the nearest residents were some 40 miles away on an opposite stretch of coastline of the Cook Inlet.

"There's nobody really living near there at all," Schneider said.

The Anchorage Police Department issued a brochure advising residents to take steps such as wearing glasses instead of contact lenses, refrain from unnecessary driving, stockpile food and water and protect electronic devices.

"There's not much we can do," said police spokesman Lieutenant Dave Parker, a veteran of the Mount Saint Helens eruption in 1980.

"When it happens it happens, and then you deal with the clean up."

The Federal Aviation Administration has issued a Notice to Airmen detailing temporary flight restrictions in the vicinity of the volcano.

Ash could disrupt air traffic from Ted Stevens Anchorage International Airport, a busy transport hub for flights arriving from east Asia.

The Atlanta-based Centers for Disease Control and Prevention meanwhile advised residents to stay indoors in the event of an eruption and to avoid driving to prevent stirring up ash.

There are 44 active volcanoes in Alaska, and Redoubt's eruption in 1989 lasted five months.

Augustine volcano, situated on an isolated island in Cook Bay on which Anchorage is nestled, erupted in January 2006 without causing any damage or casualties.

But reports of a possible eruption at Redoubt sparked so much interest that the observatory's website crashed early Friday due to the large numbers trying to access the site.

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