The volcano ejected steam followed by "a small amount of ash" in an eruption that lasted some 10 minutes at 9:47am (1647 GMT), said Tim McBicker, a spokesman for the US Geological Survey (USGS).

Although it has stopped, McBicker said "there will be additional eruptions."

Monday's eruption was similar to one on Friday that sent a huge cloud of steam and some ash into the air, USGS spokeswoman Catherine Puckett told AFP.

A tremor shook St. Helens early Sunday for 25 minutes, prompting scientists to warn of an imminent eruption two days after the volcano roared back to life.

Authorities elevated their alert level to the maximum third level over the weekend and diverted flights away from the volcano's area. Visitors were evacuated from a five-mile (8 kilometer) radius around the mountain, including the USGS Johnson Ridge Observatory.

The volcano killed 57 people during a major eruption in 1980 after weeks of quakes, slides and tremors and after belching smoke, steam and ash.

St. Helens erupted again in 1986, but with nowhere near the same intensity.

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