Alaska's Mount Redoubt continued to rumble Monday, a day after geologists at the Alaska Volcano Observatory increased the official alert level to orange, the stage just before eruption.

A significant eruption did not appear imminent, geologists said Sunday, but they cautioned that conditions could evolve rapidly.

The 3,100-metre Redoubt Volcano is about 160 kilometres southwest of Anchorage.

Ash from the volcano could harm engines and is especially dangerous for aircraft. Ash blown to cities also can cause respiratory problems.

Residents of south-central Alaska have kept a close eye on Redoubt since Jan. 25, when the observatory warned that an eruption could occur at any moment. The alert level was downgraded last week after nearly two months.

Just after 1 p.m. local time Sunday, seismic activity picked up again.

"We got a return of this stuff we call volcanic tremors," said geologist Chris Waythomas. "Think of the phenomenon that produces sound in an organ pipe."

Instead of sound waves in a pipe, geologists detect movement of magma within cracks and fractures of the mountain that resonates and produces a distinct signal.

"We think it's associated with the hydrothermal system there. It's being reinvigorated," Waythomas said.

The tremors lasted about four hours and then settled down.

Other signs that a volcano could erupt are deformities in the landscape and the mix of gases escaping from vents on the side of the mountain.

Alaska volcanoes typically explode and shoot ash upward, sometimes more than 4,600 metres, high into the jet stream. An eruption of Redoubt on Dec. 15, 1989, sent ash 240 kilometres away into the path of a KLM jet, stopping its engines. The jet dropped more than three kilometres before the crew was able to restart engines and land safely at Anchorage.