An Alaskan volcano 100 miles (160km) south-west of the state's largest city, Anchorage, erupted overnight, sending a plume of smoke up to 15,000 metres (50,000ft) into the air.
Mount Redoubt experienced four large explosions late on Sunday and early today, according to geologists at the Alaska Volcano Observatory.
The observatory has issued a red alert, meaning eruption is imminent or under way, with significant emission of volcanic ash into the atmosphere likely.
The current wind patterns are taking the ash cloud away from Anchorage and instead heading toward Willow and Talkneetna, two communities near Mount McKinley, North America's largest mountain in Denali national park.
Geophysicist John Power said no cities have yet reported any ash-fall from the volcano, but noted that it was still early.
Using radar and satellite technology, the National Weather Service (NWS) is predicting ash to start falling later today .
Dave Stricklan, a hydrometeorogical technician with the NWS, said he expected very fine ash: "Just kind of a light dusting". He said the significant amount of ash probably dropped immediately, right down the side of the volcano.
"The heavier stuff drops out very quickly, and then the other stuff filters out. There's going to be a very fine amount of it that's going to be suspended in the atmosphere for quite some time, but nothing to really affect anything such as aviation travel."
Redoubt is a steep-sided cone-shaped volcano, about 6 miles in diameter, which last erupted nearly 20 years ago.
Observatory geologists monitored the eruption and posted updates in real-time last night. At 10.35pm, they reported: "At these heightened levels of seismicity, there could be a quick escalation to eruptive activity."
Three minutes later, the first explosion took place, followed by a second about 20 minutes later. The third occurred 15 minutes after midnight and the final explosion was registered at 1.39am.
Geologists noted an increase in seismic activity at the volcano last week. Last Monday they reported a small explosion and "plume of gas and ash that rose to about 15,000ft above sea level" did not herald a significant eruption in the short term, but added: "Conditions may evolve rapidly."
Mount Redoubt releasing clouds of ash on March 15, 2009. Mount Redoubt, an active volcano west of Anchorage, Alaska, has erupted, according to the USGS Volcano Center.

Between 10:38pm on March 22 and 1:39am Alaska time on March 23, the USGS recorded four explosions at the volcano, which had been giving indications of an imminent eruption since 15 March. The USGS has issued a "code red" aviation warning and a volcano warning.

According to the Alaska Volcano Observatory, a fifth explosion began at about 4:38am Alaska time.

According to the National Weather Service, showers of volcanic ash have already been reported in Skwentna, Alaska and are possible in the Susitna Valley. Residents of those areas are advised to minimize driving, cover air intakes and open water supplies, and remain indoors.

However, Peter Cervelli of the Alaska Volcano Observatory said that "Anchorage might dodge the bullet" and come through the eruption unscathed. Dave Stricklan of the National Weather Service predicted a "light dusting" of fine ash that would disrupt air travel but not seriously affect people on the ground. Mudflows may affect the Drift and Crescent Rivers.

Mount Redoubt last erupted in late 1989, causing mudflows which put an oil refinery at risk while ash from the eruption stalled the engine of a passing airliner. US$160 million worth of damage was estimated to have been caused.