Scientists tracking renewed earthquake activity at Mount St Helens have raised their warning level to a Level 3-Volcano Alert, signaling a second eruption may be brewing.
Officials evacuated a visitor center at the Johnston Ridge Observatory about five miles from the volcano's crater as a safety precaution.
A Level 3 warning means that there is a potential hazard to life and property in the area, the US Geological Survey (USGS) said.
Mr Tom Pierson, a USGS geologist, said the alert level was raised because scientists detected low-frequency tremors, indicating that magma - molten rock underground - was moving in the volcano, which is 100 miles south of Seattle.
A second blast could top the eruption on Friday that spewed steam and ash for about 24 minutes to an altitude of 10,000 feet, he said.
Small earthquakes were detected at Mount St. Helens yesterday, a warning sign that pressure was building up again in the volcano after it belched Friday, scientists said.
"The volcano has repressurized. We think that additional steam and ash eruptions are possible at any time," Mr Dan Dzurisin, a USGS researcher at the Cascades Volcano Observatory, said.
Mount St. Helens had a catastrophic eruption in 1980 that killed 57 people, destroyed more than 200 homes, devastated hundreds of square miles, and ash drifted across North America as far east as Oklahoma.
The violent blast reduced the summit of Mount St. Helens to 8,364 feet from 9,677 feet.
Its last eruption was in 1986 and did not cause serious damage.