US: US government scientists raised the alert level for Mount St Helens in Washington state after its second steam eruption in two days was followed by a powerful tremor.
Experts said the next eruption was imminent or in progress, and could threaten life and property in the remote area near the volcano.
Hundreds of visitors at the building closest to the volcano - Johnston Ridge Observatory five miles away - were asked to leave yesterday. They left quickly, with some relocating several miles north to Coldwater Ridge Visitors' Centre, which officials said was safe.
The volcano alert was raised to level three, which "indicates we feel an eruption is imminent, or is in progress", said US Geological Survey (USGS) geologist Mr Tom Pierson from the observatory. Yesterday afternoon he said an explosion would probably happen within the next 24 hours.
Mr Pierson said the volcano had released more seismic energy since quake activity began on September 23rd than it had at any point since its devastating 1980 eruption, which killed 57 people and coated much of the north-west with ash. But scientists expect the impending eruption to be much smaller than the 1980 blast.
A day after the volcano spewed a plume of steam and ash thousands of feet into the air, there was a very brief steam release yesterday - a puff of white cloud, followed by a dust-raising landslide in the crater. A volcanic tremor signal that came next was what prompted the heightened alert.
The tremor lasted around an hour before it was drowned out by a series of earthquakes - one or two a minute, with a maximum magnitude of "well over two", said Mr Tom Yelin, a USGS seismologist.
Interior Secretary Gale Norton said the seismic activity had weakened the 1,000-foot lava dome that began forming in the volcano's crater after the 1980 eruption. The growing consensus among scientists is that new magma is probably entering the volcano's upper levels, possibly bringing with it volatile gases that could lead to eruptions, said Mr Bill Steele at the University of Washington's seismic laboratory in Seattle.
Explosions from the crater could occur without warning, possibly throwing rock onto the flanks of the volcano, the USGS said. Still, scientists said the evacuation of the observatory was primarily a precaution in case of heavy ash discharge, which could make it difficult to drive.