A Level 3 warning means that there is a potential hazard to life and property in the area, the U.S. Geological Survey (USGS) said.

Earlier on Saturday, a Level 2-Volcano Advisory was in effect for Mount St. Helens, about 100 miles south of Seattle.

Tom Pierson, a USGS geologist, told reporters that the alert level was raised because scientists detected low-frequency tremors, indicating that magma - molten rock underground - was moving in the volcano.

A second blast could top the eruption last week that spewed steam and ash for about 24 minutes to an altitude of 10,000 feet, he said.

"We're talking about more ash going up higher, tens of thousands of feet," Pierson said.

Small earthquakes were detected at Mount St. Helens on Saturday, a warning sign that pressure was building up again in the volcano after it belched Friday, scientists said.

"The volcano has depressurised. We think that additional steam and ash eruptions are possible at any time," Dan Dzurisin, a USGS researcher at the Cascades Volcano Observatory, said.

Small, shallow earthquakes were occurring at a rate of one to two a minute on Saturday, following a swarm of earthquakes before Friday's eruption.

The seismic activity trailed off after the eruption but started to rebuild late last week, the USGS said.

"The current level of seismic energy release is slightly above where it was prior to the eruption yesterday," the USGS 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.