Mount St Helens has belched more steam after several days of tremors, raising fears that one of the US's deadliest volcanoes might erupt at any moment.
Memories are still fresh of the disaster of 1980
"A small amount of ash" followed the 10-minute Monday's emission at 0947 (1647 GMT), US Geological Survey's (USGS) Tim McBicker said.
The volcano's alert was raised over the week-end to the maximum Level Three.
Scientists have said they do not expect anything close to the 1980 eruption of Mount St Helens that killed 57.
The most recent eruption at the mountain in Washington state came in 1986 - but it was nowhere near the same level of intensity and destruction.
'70% eruption chance'
"We could go into a more substantial event without warning," USGS geologist Willie Scott told reporters, after Monday's emission.
It followed a 25-minute tremor on Sunday which was gentler than nearly an hour-long shuddering recorded on Saturday.
A series of low-level earthquakes since a plume of steam was released on Friday suggested that pressure was mounting within Mount St Helens, scientists said.
"Right now, we're thinking it's about a 70% chance that it will erupt and a 30% chance that it just might go back to sleep," USGS geologist Tom Pierson told NBC's Today show on Monday.
Mr Pierson said that pressure was still mounting within the volcano.
Geological Survey crews observed a shift in the crater floor and on part of the 1,000-foot (300-metre) lava dome, which keeps down magma.
"Cracks are opening up so we know something is pushing up close to the surface right now," said Mr Pierson.
Bill Steele at the University of Washington's seismology lab in Seattle warned that the activity in St Helens could go on for weeks, saying that "no one is predicting it as a sure thing".
The volcano's level of seismic activity has prompted officials to evacuate hundreds of visitors from the nearest visitor centre - the Johnston Ridge Observatory, five miles (8km) away.
On Sunday, crowds gathered along a park road about 8.5 miles (14 km) from the mountain to wait and watch.
People lit barbecues and entrepreneurs sold hot dogs and coffee.
When Mount St Helens erupted on 18 May 1980, the upper third of the mountain was blown off.
Gray ash buried towns and cities across the Pacific North-west, and forests and meadows were devastated.