The US Geological Survey has raised its alert level to the second of three levels as small earthquakes have shaken the volcano.

"Over night, seismic activity at Mount St. Helens has accelerated significantly, which increases our level of concern that current unrest could culminate in an eruption," the USGS said in an advisory released early Wednesday.

"We have quakes that are occuring at about four per minute," Stephanie Hanna, a spokeswoman for the USGS Western Region, told AFP. "The activity has significantly increased."

The strongest quake had a magnitude of 2.5.

Mount St. Helens erupted in 1980, killing 57 people, devastating 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.

Mount St. Helens is part of the Cascade Mountains chain, which cross a large swath of land across the northwestern United States. The chain includes the Mount Adams and Mount Rainier volcanoes.

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