The chance of a major eruption of Mount St. Helens increased sharply on Saturday, as government scientists said that molten rock from deep beneath the mountain was moving toward the volcano's dome.

The alert level on the isolated mountain in southwest Washington was raised to the highest level, and about 2,500 visitors were evacuated from the Johnston Ridge Observatory, about five miles from the volcano.

A small plume of steam rose shortly after midday from the mountain -- the second plume in as many days and part of a weeklong pattern of steadily increasing seismic activity. Those rumblings, according to the U.S. Geological Survey, have now released more seismic energy than at any time since the catastrophic eruption of Mount St. Helens in the spring of 1980.

Interior Secretary Gale A. Norton, traveling in the Pacific Northwest this weekend and briefed by USGS scientists, said that "the greatest public safety concern" from an eruption "is an ash plume and the spread of ash itself."

She said that an eruption could launch rocks as much as a foot in diameter as far as three miles from the volcano's core, but that this would present little risk because people have already been moved well away from that danger zone.

In a statement Saturday afternoon, the USGS warned that if there were an explosion, "the rim of the crater and flanks of the volcano could also be at risk." A large explosion, it said, could send a cloud of ash tens of thousands of feet into the air, and it could drift downwind over population centers.

The National Weather Service forecast winds moving to the northwest this weekend, which could move clouds of ash toward the cities of Kelso and Longview, but probably well south of the Seattle-Tacoma area, the major population center in Washington.

After Saturday's steam eruption, scientists noticed a new pattern of tremors in the mountain. It prompted them to change their opinion of what was causing the recent increase in seismic activity, Tom Pierson, a USGS geologist, told reporters.

Instead of smoldering "older magma" from previous volcanic activity, he said he and his colleagues now believe that "fresh magma" is moving up inside the volcano and that it is responsible for the "harmonic tremor" that was detected on the mountain for about 50 minutes on Saturday afternoon.

"We saw something we have not seen on Mount St. Helens before," Jon J. Major, a USGS hydrologist, said in an interview. "Previous earthquakes had been intermittent and suggested the crushing of rock. Now we are seeing a type of earthquake that is indicative of magma moving inside the mountain."

Major also said that seismologists have, for the first time in the mountain's current round of seismic activity, detected releases of hydrogen sulfide, which he said also suggests that magma is working its way toward the surface.

"It is possible that an explosion is imminent," said Major. Pierson said it could occur within 24 hours.

By raising the alert to "level three," the USGS was warning that an eruption could threaten life and property near the volcano. But the greatest threat on Saturday appeared to be to sightseers and journalists. By late afternoon Saturday, they had been moved at least 7 miles away from the mountain.

The 1980 eruption on Mount St. Helens killed 57 people, triggered the largest landslide in recorded history and launched more than 500 million tons of ash eastward across the United States.

That eruption ripped the top off the mountain, and scientists have said repeatedly that even a major new eruption is unlikely to cause so much damage -- in part because there now is much less mountain to blow up.