Mount St. Helen's dramatically exploded in May 1980, devastating the northwestern United States and killing 57 people.

The alert was raised to level three out of a maximum of four around the volcano in the US state of Washington, as an ominous flurry of earthquakes increased in the area after a week of growing rumblings from the earth.

Seismologist Jeff Wynn of the US Geological Survey said the flurry quakes were taking place four times a minute with magnitude ranging between 3.0 and 3.2 on the Richter Scale, raising the probability of an eruption.

"The science team did a probability tree and are announcing a 70 percent likelihood of an eruption," he said, adding however that there was no telling whether such an explosion would occur within days or months.

"The most likely scenario at this stage is an ash-steam eruption with ash going to several to tens of thousands of feet (meters).

"If there is an eruptive event, it could involve ballistic projectiles going as far as five kilometers (three miles) from the dome (of the volcano), likely in a northward direction," Wynn said.

But while children in the area that straddles the border with Oregon practiced evacuation drills, officials said that any eruption that did occur would not resemble the deadly 1980 explosion.

The area north of the peak was hard hit in Mount St. Helens' explosion 24 years ago that devastated hundreds of square kilometers (miles) and spewing ash over much of the Pacific Northwest region.

The mountain's top lies around 88 kilometers (55 miles) northeast of the Oregon's main city of Portland, which was also covered in a thick layer of ash in the 1980 eruption.

"The seismic activity has increased a little bit, but that doesn't tell us anything more about how close or how far a way we are from something," USGS seismologist Seth Moran told reporters.

"But there is about a one in three chance that whatever is happening right now could stall out and we could start getting some sleep again," he said.

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