Seattle - Mount St Helens spewed more steam and ash on Monday, raising concerns about a larger eruption at the Washington state volcano that woke last week after 18 years of slumber.
Mount St Helens, which in 1980 killed 57 people when it erupted violently, increased its activity after a week of tremors and a similar steam and ash eruption on Friday.
"We could go into a more substantial event without warning," Willie Scott, a US Geological Survey geologist, told reporters.
Scott said that the lava dome that formed in the crater after the 1980 eruption, as well as a glacier nestled next to it, had risen 50 to 100 feet (15-30m), a hint that magma was pushing up from underneath.
The 1980 eruption was one of the most devastating by the volcano in modern history. In addition to the heavy casualty toll, it destroyed more than 200 homes and flattened evergreen spruce forests. It continued to have a series of smaller eruptions until 1986.
The US Geological Survey maintained its warning at a Level 3-Volcano Alert, the highest level, and kept a nearby visitor centre at the Johnston Ridge Observatory off limits as a safety precaution.
Government scientists said that low-frequency earthquakes, carbon dioxide and hydrogen sulfide gas, and swelling in the lava dome all pointed toward the build-up of magma and pressure under the mountain.
Although scientists have said that they do not expect an explosion that would cause any deaths, they are concerned about the impact of any ash on people's health and on air traffic.
Mount St Helens is about 160km south of Seattle, and 80km north of a busy airport at Portland, Oregon.
A thick plume of steam rose from the crater of Mount St.Helens on Monday at around 9.30am. (16h30 MT), after a similar discharge 11 hours earlier.
The US Federal Aviation Administration also notified nearby aircraft of Monday's steam eruption, and directed them to avoid the steam and ash plume, said Mike Fergus, an agency spokesman. Aircraft engines can be stalled by ash.
Winds were blowing toward the east from Mount St Helens at five to 18-24km/h toward sparsely populated areas, meteorologists said.
Dr Justin Denny, health officer at Clark County, Washington, just west of where Mount St Helens is located, said people with breathing problem could be affected by the ash.
"If you have to be out in the ash, goggles and masks are needed," Denny advised.
The violent blast in 1980 blew off the top of the mountain and reduced the summit of Mount St Helens to 2,550m from 2,950m.
Smaller eruptions in the lava dome happened in 1986 but caused no serious damage.