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."
Le Mont Redoubt le 15 mars 2009
 Le Mont Redoubt, un volcan actif situé à l'ouest de la ville d'Anchorage en Alaska, est entré en éruption, selon l'USGS Volcano Center. Ce centre a enregistré quatre explosions entre 22H38 (22 mars) et 1h39 (23 mars) heure locale. Il a également émis une alerte volcanique et un code rouge pour l'aviation. Selon l'Alaska Volcano Observatory, une cinquième explosion a commencé à 4:38. Des cendres volcaniques sont tombées sur Skwentna et sont possibles sur Susitna Valley, d'après le National Weather Service. Les habitants de ces régions sont invités à réduire leurs déplacements et à rester chez eux. La dernière éruption du Mont Redoubt remonte à la fin 1989 : elle provoqua des coulées de boue qui avaient menacé une raffinerie. Elle éjecta des cendres volcaniques jusqu'à 14 000 m (45 000 pieds) d'altitude et un Boeing 747 de la KLM Royal Dutch Airlines fut pris dans son panache de cendres (le vol réussit à se poser à Anchorage). Les cendres couvrirent une zone d'environ 20 000 km².